Access switches for large ATM networks

ABSTRACT

Pulse Code Modulation (PCM) signal streams are converted into Asynchronous Transfer Mode (ATM) cells for switching and transmission across a telecommunications network. Each cell carries one PCM sample of up to 48 different voice connections, the voice connections being selected from the PCM data streams because they have a common destination. The cells are transmitted over ATM virtual paths, each path transmitting one cell every 125 μs. The ATM signals are switched by units for switching cells (cell switching), while keeping their payload contents intact, and by units for switching the signals for individual voice connections between cells of an ATM signal (cell remapping). Advantageously, ATM transmission systems interface with PCM systems without adding appreciable delay and without requiring additional buffering. Advantageously, new voice paths can be established most of the time by using available slots in the cells of existing virtual paths. Advantageously, large telecommunication networks can be implemented using a small number of switching units. 
     Advantageously, an access switch comprising a cell mapping or distribution stage for assembling cells comprising PCM samples headed for a common output of the access switch are switched in a cell switch to a cell remap unit for collecting PCM samples headed for a common destination into individual cells of an ATM output stream, provides output signals that can be readily and efficiently switched by a transit network for distribution to other access switches of a large network.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to the applications of:

Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled"Establishing Telecommunications Call Paths In Broadband CommunicationNetworks", Ser. No. 07/972,789, filed Nov. 6, 1992, now U.S. Pat. No.5,345,446;

Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled"Establishing Telecommunications Call Paths Between Clustered SwitchingEntities", Ser. No. 07/972,787, filed Nov. 6, 1992, now U.S. Pat. No.5,365,524;

Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled "ApparatusFor Interfacing Between Telecommunications Call Signals And BroadbandSignals", Ser. No. 07/972,786, filed Nov. 6, 1992, now U.S. Pat. No.5,327,421;

Thomas L. Hiller, James J. Phelan, and Meyer J. Zola entitled"Establishing Telecommunications Calls In A Broadband Network", Ser. No.07/972,788, filed Nov. 6, 1992, now U.S. Pat. No. 5,345,445; and

Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.Wierzbicki, and Meyer J. Zola entitled "ATM Networks For Narrow BandCommunications ", Ser. No. 08/170,549, filed Dec. 20, 1993,

Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.Wierzbicki, and Meyer J. Zola entitled "ATM Distribution Networks ForNarrow Band Communications", Ser. No. 08/169,913, filed Dec. 20, 1993;filed on an even date herewith,

Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.Wierzbicki, and Meyer J. Zola entitled "Inter-Cell Switching Unit ForNarrow Band ATM Networks", Ser. No. 08/170,550, filed Dec. 20, 1993, nowU.S. Pat. No. 5,390,175, filed on an even date herewith and

Thomas L. Hiller, Ronald A. Spanke, John J. Stanaway, Jr., Alex L.Wierzbicki, and Meyer J. Zola entitled "Intra-Switch Communications InNarrow Band ATM Networks", Ser. No. 08/169,915, filed Dec. 20, 1993,filed on an even date herewith,

which applications are assigned to the assignee of the presentapplication.

TECHNICAL FIELD

This invention relates to arrangements for establishing digitaltelecommunications connections, and more specifically, for establishingsuch connections using broadband networks and switching systems.

PROBLEM

The provision of large quantities of toll telephone service continues tobe expensive. While there have been major breakthroughs in the cost ofprovisioning of high capacity transmission systems using fiber optics,the access to such fiber optic systems and the switching of signals onsuch fiber optic systems continues to be costly. Further, the cost ofswitching the signals from one channel to another in a tandem-tollswitch remains high.

Further, there has been an increasing need for a very largetelecommunications switching system or its equivalent. In the past, thisneed has been partially met by the use of smaller switching systemsinterconnected by moderate capacity tandem switching systems. In thecase of a switching system such as AT&T's 5ESS® switch, a fairly largesystem has been devised using switching modules of substantial capacityinterconnected by a time multiplexed switch. None of the availablesolutions, however, have resulted in an economically satisfactorysolution to the need for a very large switching system or cluster ofsystems for simultaneously handling substantial quantities of telephonetraffic, low speed telecommunications data traffic, and high speedtelecommunications data traffic.

A new standard has been implemented for transmitting combinations ofbroadband and narrower band, packet and circuit signals over broadbandfacilities. This standard, the Asynchronous Transfer Mode (ATM)standard, packs data into frames each frame comprising a plurality ofcells, each cell being 53 bytes long, the 53 bytes consisting of a 5byte header and a 48 byte payload. When a 125 microsecond ATM frame istransmitted, each of the cells may be headed for a separate destination,the destination being identified in the cell header. ATM standards underconsideration specify how to pack 48 voice samples from one speechchannel into a cell and to transmit this cell across an ATM network.However, this incurs an undesirable 48 sample (6 millisecond) delay forfilling the cell with the samples, and requires a large amount ofstorage for the samples prior to their transmission and after theirreception. No sound proposal has been made public for the economic useof ATM for transmitting voice signals from an ingress node to a networkto a multiplicity of egress nodes of the network without incurring thisdelay. While the ATM standard is finding increasing use in broadbandnetworks, especially those using fiber optic transmission facilities, noeconomic solution has been made public to the problem of designing acommunications network for transmitting a large multiplicity ofnarrowband voice signals from any of a multiplicity of ingress nodes ofthe network to any of a multiplicity of egress nodes of the networkusing the ATM standard without incurring this delay. Further, no soundeconomic proposal has been made public for the economic use of ATM forachieving a very high capacity low delay large switching system or alarge highly interconnected cluster of smaller switching systems.

The above problems are partly solved in accordance with the teachings ofa prior patent application Ser. No. 07/972,789, and its relatedapplications, Ser. Nos. 07/972,787, 07/972,786, and 07/972,788.According to the teachings of those applications, signals from aplurality of pulse code modulated (PCM) channels on a plurality of PCMlines, each channel for one telecommunications call, are switched by ATMinterface units (ATMUs) onto ATM signal outputs such that each of thecalls destined for a common switching module or independent switchingsystem are packed into a single composite ATM or ATM-like cell; the ATMsignals containing such composite cells are transmitted to and from anATM Communication module (ATM-CM) (called a common broadband platform inthe cited application) which is used for switching ATM cells; suchcomposite cells are transmitted at a repetition rate that is the same ora sub-multiple of the repetition rate of the PCM signals that representthe voice signals; the cells are transmitted over constant bit rate(CBR) permanent virtual circuits (PVC) from an ingress switching moduleor system to the ATM-CM of that node or access switch, thence, to anegress switching module or switching system or to the ATM-CM of anotheraccess switch. This solution avoids the 6 millisecond delay because onlyone sample from a given call is placed into a composite cell. Permanentvirtual paths are provisioned as the traffic between a particularingress and egress module changes, but such paths need to be activatedor deactivated only when an additional group (the group size beingdetermined by the number of voice channels that are transmitted in eachcell) is needed or can be released.

A problem with this partial solution is that there is insufficienttraffic to justify permanent virtual paths among the large plurality ofATMUs in a network having many switching modules. Further, the partialsolution has the disadvantage that the addition of a single switchingmodule to any access switch of a network requires that all accessswitches be informed, thus presenting a significant operational andadministrative problem.

SOLUTION

In a departure from the prior art, described in this application, acomposite cell remap (CCR) unit is introduced between ATM cell switchingstages. The access switch is enhanced to include a plurality of CCRunits, each of which converts between cells on an ATM signal receivedfrom an ATM-CM, each cell being from one of a plurality of sourceswitches or switch modules and destined for that CCR unit, to cells eachcontaining communications destined for a common destination CCR; this isaccomplished by switching the individual channels within each cell ofthe input ATM signal to cells headed for the appropriate destination CCRin the output ATM signal of the CCR. A transit crossconnect isinterposed between CCRs of different access switches, to switch cells ofincoming ATM streams to different outgoing ATM streams. This transitcrossconnect is equivalent to the network of crossconnects used today inlong distance synchronous voice networks. Advantageously, using thiskind of an arrangement, a transit crossconnect, in conjunction with aplurality of CCRs in an access switch, can be used efficiently tointerconnect the links of a permanent virtual path between two accessswitches, and can link modules of different access switches even whenthe traffic between the modules is well below the capacity of a singlecell.

In accordance with one specific embodiment, each composite ATM cellbetween CCRs carries one byte of each of up to 48 voice communications,and the composite cells of the CBR PVCs are transmitted at a rate of onecell per 125 microseconds (μs). Advantageously, such an arrangementsimplifies the interface to existing PCM systems.

Virtual paths in the ATMU are assigned to gather traffic that is inputto the ATMU into cells such that each cell goes only to one CCR or ATMUdestination. For intra-access switch traffic the cells go to the ATMUconnected to the destination switch module. For inter-access switchtraffic, the cells go to a CCR and are then transmitted over an ATM linkto a transit crossconnect or directly to another access switch. Theaccess switch that contains the destination CCR, or for intra-accessswitch traffic, the destination ATMU, determines the choice of whichinputs are assigned to which cells. In the case of cells destined for aCCR, if the CCR is directly connected to another access switch, asituation which may occur when either there is a great deal of trafficbetween two access switches or when networks are initially small (i.e.,few access switches), then the traffic that can be placed in a cell isany traffic destined for the terminating access switch. For the moregeneral case, in which the CCR that is the destination of the cell isconnected to a transit crossconnect, the cell from the ATMU may containtraffic for any of the access switches connected to the transitcrossconnect; the CCR will serve to segregate traffic for each of thepossible destination access switches of the connected transitcrossconnect into different cells. The transit crossconnect routes eachcell to the appropriate destination access switch. There, another CCRunit will segregate traffic for each ATMU of that access switch intodifferent cells. Each cell is then routed through the destination accessswitch ATM-CM to the correct ATMU for that cell, and thence to thedestination switching module.

The assignment of traffic to cells, i.e., the assignment of traffic tothe virtual paths of the ATMU, is further modified by considerations ofreliability so that, whenever possible, traffic between access switchesmay be switched by a network of crossconnects, e.g., via two differenttransit crossconnects. If the amount of traffic between two accessswitches is small, then it may normally be carded over one of the twotransit crossconnects and in case of system failure may then be switchedto a CCR connected to the second transit crossconnect interconnectingthe two access switches.

The assignment of traffic to output cells within the CCR is relativelystraightforward. If the CCR is connected to a transit crossconnect, eachoutput cell contains traffic destined for a single destination CCR of adestination access switch. Because the destination access switch alsocontains a CCR unit and because of the full interconnectivity of theATM-CM in the destination access switch, there is no need to segregatetraffic destined for switching by different ATMUs in the destinationaccess switch.

All of the units described herein carry traffic in both directions. Ingeneral, only the outgoing direction is described in detail. Thoseskilled in the prior art will readily understand the adaptationsnecessary for the reverse direction of transmission and switching.

While the preferred embodiment uses ATM, the principles of applicants'invention apply to any packet network. Implementation is simplified bythe constant length packet size of the ATM cells and the use of ATMstandard parts will make implementation of applicants' inventioneconomical. The equivalent of an ATMU and a CCR in a more general packetnetwork are units which can pack and unpack the contents of the packetsover the period of a frame of a multiplexed periodic communicationsignal such as PCM or PCM-like (e.g., adaptive PCM) signals. Theequivalent of a CCR in a more general packet network is a compositepacket remap (CPR) unit. The equivalent of an ATM crossconnect ornetwork of crossconnects is a packet switching fabric unit, or networkof packet switching fabric units.

While the above principles are used in all five related applicationsbeing submitted concurrently, the particular subject claimed in thisapplication relates to the access switch including at least one ATMU, anATM-CM and at least one CCR. Advantageously, such an access switch cangather traffic from a large number of sources and bundle this trafficinto composite cells carrying PCM samples for a plurality ofcommunications wherein each of the composite cells at the output of anaccess switch contains traffic destined for a single destination accessswitch. The access switch can also be used advantageously for carryingconventional packetized voice (see FIG. 12) or packetized data from andto the switches and modules that are part of or are connected to theaccess switch.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1-4 are high-level network configuration diagrams whoseconfiguration is in conformance with the principles of this invention;

FIG. 5 is a functional diagram of central stages of a network, an ATMdistribution network, designed in conformance with the principles ofapplicants' invention;

FIG. 6 is a block diagram showing a network of interconnected accessswitching networks;

FIG. 7 illustrates the connections to the access switches of such anetwork;

FIG. 8 illustrates one 125 μs frame of ATM cells (a glossary ofabbreviations is found at the end of the Detailed Description)comprising a plurality of CBR and a plurality of CBR cells; the constantbit rate (CBR) cells carrying voice channels are sent every 125 μs;

FIG. 9 illustrates an ATM segment including a CBR cell and a variablebit rate (VBR) cell;

FIG. 10 illustrates one CBR cell for carrying traffic that is initiatedfrom both of the two access switches communicating via the CBR cell;

FIG. 11 illustrates the Synchronous Optical Network (SONET)/ATM signaltransmission/crossconnect network of FIG. 6;

FIG. 12 illustrates the access switch structure and the position of acomposite cell remap (CCR) function relative to the communicationnetwork;

FIG. 13 presents an overview of how cells are transmitted from a sourceswitch module to a destination switch module;

FIG. 14 illustrates a single destination composite cell format of thetype which would be used for communications between switching modules ofone ATM-CM switch;

FIG. 15 illustrates a multiple destination composite cell (MDCC) formatof a type used for communicating between a switching module and acomposite cell remap unit;

FIG. 16 illustrates the format of a cell used for communicating betweenCCR blocks;

FIG. 17 illustrates a tandem destination cell arranged to communicatewith a packetized voice destination;

FIG. 18 illustrates the mapping performed between incoming time slots toan ATMU and an SDCC/MDCC (Single Destination Composite Cell/MultipleDestination Composite Cell) cell;

FIG. 19 illustrates the remapping function performed in the CCR;

FIG. 20 illustrates the overall mapping process from source SwitchModule (SM) to destination SM for communications using a composite cellremap function;

FIG. 21 illustrates the signaling used between ATM switches and betweenATM and between toll and local exchange carrier switches;

FIG. 22 is a block diagram of an ATM switch for supporting bothbroadband and narrow band ATM communications in conformance withapplicants' invention;

FIG. 23 is a block diagram of an access switch of FIG. 6;

FIG. 24 is a block diagram of an Asynchronous Transfer Mode InterfaceUnit (ATMU) for interfacing between PCM signals and ATM signals;

FIGS. 25-29 illustrate various blocks of the ATMU;

FIG. 30 illustrates the control complex of an ATM Communications ModuleATM-CM unit for switching ATM cells;

FIGS. 31-33 are flow diagrams illustrating the processes of selecting achannel for a communication activating permanent virtual circuits andcombining traffic of partially loaded permanent virtual circuits;

FIGS. 34-37 illustrate the operation of a call at the originating end;

FIGS. 38-41 illustrate the operation of a call at the terminating end;

FIGS. 42-45 are block diagrams of four versions of a CCR; and

FIG. 46 is a block diagram of an address generator for a CCR.

OVERVIEW

The telecommunications network described herein uses, packets, and inthe preferred embodiment ATM cells, to switch PCM sources supplied fromlocal switches. According to the principles of this invention, which canbe followed in FIG. 12 for the purposes of this overview, the networknot only uses ATM signals as a broadband means of transporting PCMsignals and of switching different PCM streams to different destinationsthrough the use of an ATM switch fabric capable of switching differentcells to different ATM destinations, but also includes facilities forswitching different PCM samples within a cell to other cells. The resultis that PCM streams, each containing communications destined for a largenumber of different destinations, can have their contents rearranged andswitched among a plurality of ATM streams each containing cells destinedfor different destinations. ATM cell switches are described in Journalof High Speed Networks, Vol. 1, No. 3, pp. 193-279, 1992.

In accordance with the principles of this invention, input PCM streamsfrom one or more switching modules 510 are transmitted to an ATMinterface unit (ATMU) 540 of FIG. 23. In the specific embodiment thesePCM streams are sent on Network Control and Timing (NCT) links and aregenerated by a switching module of a 5ESS switch which has inputs (notshown in FIG. 12) from local switches and from telephone stationsconnected to the switching module. The 5ESS® switch and switching moduleare extensively described in the AT&T Technical Journal, Volume 64, No.6, July/August 1985, pp. 1303-1564. The ATMU takes PCM streams from aplurality of such switching modules and generates ATM streams eachcontaining cells, where for each cell all of the PCM samples aredestined for a common composite cell remap (CCR) unit 4000 (FIG. 42) ofthe source access switch 1. ATM streams from a plurality of ATMUs aretransmitted to an ATM Communications Module (ATM-CM) 550, comprising anATM switch fabric which switches cells from that plurality of ATMstreams into a plurality of CCR units. (Remember that each cell of anATMU output is destined for a particular CCR and contains only PCMsamples for that CCR.)

The output of the CCR is a single ATM stream which is transmitted to atransit ATM crossconnect 600 (FIG. 11 ) which, like the ATM-CM, switchescells among different ATM streams, but keeps the payload content of eachcell intact. The CCR performs the function of re-packaging the cells ofits input stream so that each cell in its output stream contains PCMsamples destined for a single CCR unit at a destination access switch.The transit crossconnect can then take the inputs from a plurality ofCCRs and gather those cells destined for a CCR in a particulardestination access switch into a single ATM stream for transmission tothat destination access switch CCR. The transit crossconnect switchesthe cells in its input ATM streams but does not switch parts of thepayload data of one cell to two or more different cells.

At the destination access switch, the process is simply reversed. TheCCR gathers samples destined for a common ATMU into cells and transmitsits output stream to an ATM-CM. The ATM-CM sends the cells received froma plurality of CCRs of the destination access switch and destined for aparticular ATMU to that ATMU. That ATMU then forms PCM streams to itsconnected switching modules from the contents of the ATM streams that ithas received. The ATMU can take individual PCM samples from within thecells of its input ATM streams and direct these samples to different PCMoutput streams. In this preferred embodiment, the ATMU generates andreceives a plurality of ATM streams, while the CCR operates on only oneATM stream; clearly this is a matter of design choice and the ATMU canbe arranged to interface with a single ATM stream and/or the CCR can bearranged to interface with a plurality of ATM streams.

For the case in which the access switch acts as a tandem switch betweenlocal offices connected thereto or an intraoffice call between linesdirectly connected to switching modules of the access switch, the ATM-CMacts to connect output streams of an ATMU to input streams of the sameor another ATMU. For compatibility reasons it is desirable in this casethat additional information associated with each PCM sample, in thiscase a supervisory indicator or E-bit, be switched through the ATMU inboth directions. The ATMU in order to provide for this capability in thepreferred embodiment deals with 9 bit segments, each segment includingan 8 bit PCM byte and an E-bit associated therewith and 42 of these 9bit segments make up the payload of each ATM cell in the ATMU. Forconvenience, this is done whether or not the cell is connected to a CCRfor further switching and subsequent transportation to another accessswitch. Advantageously, using this arrangement the E-bit which istransported over NCT links connecting a switching module to an ATMU isthereby retained in all connections which do not leave the accessswitch. Clearly, this is a matter of design choice and the ATMU and CCRcould be designed to work with different size segments and the number ofsegments adjusted accordingly to fit into a 48 byte ATM payload. Whilein the preferred embodiment, only the E-bit is switched through in anintra-access switch connection, it is obvious that additional bits canbe switched through using the same principles. In some applications,other bits may be transmitted on a sampling basis or can be transmittedvia signaling messages.

FIGS. 1-5 present a high-level overview of telecommunications networksdesigned in conformance with applicants' invention. In all cases, in theillustrative embodiment, the inputs are PCM streams and the outputs arePCM streams.

FIG. 1 is based on the teachings of the prior patent application Ser.No. 07/972,789. It shows the treatment of an intra-access switch call. APCM stream from, for example a 5ESS switch enters an ATMU wherecomposite cells are generated, each composite cell containing PCMsamples for destinations served by one ATMU. The cells are switched inan ATM cell switch, the ATM-CM 550, where inputs from a plurality ofATMUs 540 are switched to a plurality of outputs to ATMUs 540. TheATM-CM does not change the payload within each cell, but simply switchesindividual cells on one ATM input stream to one of a plurality of ATMoutput streams. These output streams are received in ATMU units 540acting, in this case, as ATM composite cell receivers and the individualPCM samples of each received cell are distributed to appropriatepositions within the correct PCM output stream.

FIG. 2 illustrates the case of a two-stage network using compositecells. This is a network for switching PCM samples on a plurality of PCMinput streams on one access switch to PCM samples on a plurality of PCMoutput streams on a different access switch, wherein the two accessswitches are directly interconnected via a link carrying one or more ATMstreams. The PCM input streams enter a composite cell generation ATMU540 which assembles cells all of which are destined for the same CCR, aCCR which, in this case, is directly connected to a single destinationaccess switch. The ATMU 540 of the first (top) access switch effectivelygenerates composite cells destined for the second access switch. Thesecomposite cells are transmitted over one or more ATM streams to ATM-CM550 where cells from a plurality of ATMUs entering the ATM-CM arecombined into a single ATM stream for CCR 4000, which is connected tothe second access switch. The CCR remaps the PCM samples within thecells such that all PCM samples within the same output cell are destinedfor the same ATMU in the destination access switch. In the second(bottom) access switch, the output of the CCR 4000 from the first accessswitch can be directly connected to an ATM-CM 550, which separates thecells destined for each of the ATMUs 540 of the second access switch.Each of these latter ATMUs then converts its input ATM streams into PCMstreams in essentially the way previously described for the compositecell reception in FIG. 1. A problem arises in that the switch containingthe CCR needs to know about the ATMUs of the other access switch. Onesolution to this problem is to exchange data messages between the accessswitches. Another is to equip only incoming CCR portions in each accessswitch. It is also possible to equip CCRs in both access switches.

FIG. 3 illustrates the network configuration most likely to beencountered and the one that is described most fully in thisspecification. It illustrates the case of two access switchescommunicating via a transit crossconnect. In the first (top) accessswitch, the ATMU 540 acts as a composite cell generation unit whichgenerates individual cells, each of which contain samples headed for acommon CCR unit 4000 of the first access switch. The ATM-CM of the firstaccess switch assembles all cells destined for that CCR into a singleATM stream which it transmits to CCR 4000. CCR 4000 maps the PCM samplesfrom different incoming cells, destined for a single access switch, tooutgoing cells each containing only PCM samples destined for one accessswitch. The output of CCR 4000 is transmitted to a transit crossconnect600 which collects all cells destined for a common destination accessswitch into an ATM stream and transmits that ATM stream to the CCR 4000of the second (bottom) (destination) access switch. The switchingfunction executed by ATM crossconnect 600 is the same as the switchingfunction executed by ATM-CM 550. In the destination access switch, theCCR remaps cells so that each cell contains PCM samples destined for asingle composite cell reception ATMU 540. The output stream of CCR 4000is sent to ATM-CM 550 of the destination access switch, where cellsdestined for a common ATMU are gathered into ATM streams fortransmission to that ATMU. The ATMU then receives its input ATM streamsand distributes the individual PCM samples contained in the stream toits PCM outputs. While FIG. 3 shows only a single ATM cell switch(transit crossconnect), in practice a network of such crossconnects islikely to be used.

Finally, FIG. 4 illustrates a connection wherein the path through thetransit network interconnecting the source and destination accessswitches includes two transit crossconnects connecting through a CCR.The actions in the source (top) access switch are essentially the sameas described with respect to FIG. 3, except that in this case, theoutput of CCR 4000 of the source access switch may include cells whichcontain samples destined for a plurality of destination access switches.Presumably, this would be a case in which there is low traffic to eachof the destination access switches served by this type of connection.First, the fight ATM transit crossconnect 600 receives inputs from aplurality of access switches and transmits those cells destined forreswitching to transit CCR 4000 interconnecting the two ATM transitcrossconnects. In transit CCR 4000, samples destined for a commondestination access switch are assembled into cells to be switched by thesecond (left) transit crossconnect of the transit network to thatdestination access switch. A second (left) ATM transit crossconnect thenreceives inputs from CCR 4000, as well as direct inputs from otheraccess switches, and generates output ATM streams, each streamcontaining only cells with PCM samples destined for a common destination(bottom) access switch (as well as output streams destined for the ATMinter-transit crossconnect CCR 4000 of the same type discussed as beingreceived by the second ATM transit crossconnect). In the destinationaccess switch, the same types of operations are performed as in thedestination access switch of FIG. 3. The middle CCR receives specialcontrol signals from the overall network control or, following exchangesof messages between the two access switches, receives control signalsfrom one of the two, to establish virtual paths and circuits using thatCCR. The configuration of FIG. 4 can also have connections between thetwo transit crossconnects 600 in order to allow some of the CCR outputcells to return to a crossconnect that may also be connected to adestination access switch. (Such a connection is shown as connection 7in FIG. 11.) As previously stated for FIG. 3, the transit crossconnectsmay each represent an interconnected network of such crossconnect units.

While FIGS. 1-4 and many other diagrams of this specification show onlya single direction of flow of information, it is understood that acomparable opposite flow of information is simultaneously taking placein essentially the same way. Each ATMU, for example, acts both as acomposite cell generator and as a composite cell receiver. Similarly,each CCR performs its remap function in both directions.

FIG. 5 presents another view of the functions of the CCR and the ATMcrossconnect. The input to the CCR comprises cells, each of which mayhave samples destined for a plurality of the CCRs at the output of anATM crossconnect 600. The input CCRs gather these samples into cells,each of which contains samples destined only for a single output CCR.These cells are placed on an output ATM stream which enters the ATMcrossconnect 600. The ATM crossconnect 600 then switches individualcells from all the incoming ATM streams to outgoing ATM streams suchthat the outgoing ATM streams each contain cells only destined for theparticular CCR to which the output stream is connected. The output CCRthen takes individual cells, each of which may contain samples for aplurality of the ATMUs of the access switch of which the output CCR is apart, and creates cells each of which contain samples destined for onlyone of these ATMUs. These cells are then subsequently switched in theATM-CM of the output access switch to the appropriate ATMU of the outputaccess switch. An ATM-CM 550 is basically an ATM crossconnect, enhancedto provide interfaces with other units as shown in FIG. 22. Adistribution "network" consisting of a plurality of CCR unitsinterconnected by one or more crossconnects is a very useful element forswitching narrowband signals carded by ATM streams. In the preferredembodiment, such a distribution network accepts as its inputs ATMsignals containing cells carrying PCM channels destined for any accessswitch accessible via the transit crossconnect connected to the outputof the CCR (and in the case of a CCR directly linked to another accessswitch, the access switch to which the latter CCR is connected). Thecontents of the input cells to these CCRs are switched into cells eachof which contains communications destined for a common access switch.The output cells of these CCRs are then switched in the transitcrossconnect to the ATM streams connected to that destination CCR wherethe ATM signal is switched into cells each of which are destined for thesame ATMU in the destination access switch. Advantageously, thecombination of the CCRs and the transit crossconnect yields adistribution network of enormous capacity with low blockage for servinga large number of very high capacity access switches. In theory, theCCRs can be associated with either the access switches or the transitnetwork. In the preferred embodiment, the control information requiredby the CCR comes naturally from the control of the access switch,thereby making a co-location of access switch and CCR more natural.

Advantageously, in such an arrangement permanent virtual paths can beestablished through the transit crossconnects for composite cells eachcarrying individual PCM samples for a plurality of PCM channels. Suchpermanent virtual paths can be pre-provisioned and need be activatedonly when an additional group of channels is required for a particularsource/destination couplet of the transit crossconnect and aredeactivated only when there is a substantial reduction in the need forthe number of channels required of permanent virtual paths between sucha source/destination pair of the transit crossconnect.

GENERAL DESCRIPTION

This General Description first presents an overview of all of thediagrams and is followed by a detailed description of specialcharacteristics of elements of these diagrams for implementingapplicants' invention.

FIG. 6 is a block diagram showing a plurality of interconnected accessswitching systems of a network. A group of access switches 1 areconnected to an ATM signal transmission/crossconnect network 10 inaccordance with the principles of this invention. Such a network is anetwork of ATM crossconnects for interconnecting a plurality ofComposite Cell Remapping (CCR) units and for switching cells of the ATMstreams generated by the connected access switches. Each access switchcontains a CCR that receives an ATM signal from the ATM-CM of the accessswitch, comprising a plurality of Constant Bit Rate (CBR) compositecells each carrying signals destined for the network, i.e., for a CCRconnected to the network, and distributes the individual PCM channelsignals carried in each input cell to a CBR composite output cell havinga common access switch as a destination. The composite output cell isthen switched in the network of ATM crossconnects to a CCR of thatdestination access switch. The access switch contains an ATM interfaceunit (ATMU) for forming composite cells from PCM samples of inputs tothe access switch and sending the composite cells on ATM signals to anATM Communications Module (ATM-CM) which switches the individual cellsof the ATM signal from the ATMU to the correct CCR of the access switch.Each composite cell of the output of an ATMU is destined for a singleCCR of the access switch or for the same or for inter-access switchcommunicating for another ATMU of the access switch. Constant bit rate(CBR) cells are used to carry PCM voice traffic, and variable bit rate(VBR) cells are used to carry packetized data. (The term PCM as usedherein refers both to voice signals transmitted by PCM and to data(including facsimile (FAX) and video) transmitted over PCM channels.)

Throughout this specification, there are frequent discussions ofactivation of permanent virtual circuits and permanent virtual paths. Apermanent virtual path, comprising a plurality of permanent virtualcircuits, is provisioned by including its identification in the memoryof an ATMU, ATM-CM, CCR or ATM crossconnect. The memory is used forswitching cells having the virtual path identification as their header,in the ATM-CM and ATM crossconnect and for identifying the cells to theATMU and CCR for their operations. The virtual circuit information isretained in the ATMU and CCR in order to steer PCM samples between thecorrect cells or between the correct cells and correct PCM stream andposition in that stream. A virtual path is activated only when actuallyused to carry communications. The number of provisioned permanentvirtual paths can therefore be much greater than the number of activatedpermanent virtual paths. A permanent virtual circuit is similarlyactivated when the specific circuit (or channel), of an activatedpermanent virtual path, is used. If a cell of a virtual path is full andanother channel is needed, then another, previously provisioned,permanent virtual path is activated to carry that channel over a virtualcircuit of the newly activated permanent virtual path. Note that this isinconsistent with standard ATM terminology, but such terminology doesnot have to deal with composite cells for carrying a plurality ofcommunications. Applicants have chosen to call the byte or segmentposition of a cell for carrying each such communication a virtualcircuit since it carries, essentially, a circuit switched communication.

The access switch and transmission/crossconnect network can also be usedfor switching and transporting conventional packetized voice asillustrated in the cell of FIG. 17. The disadvantage of conventionalpacketized voice is that it incurs additional packetizing delay and itrequires expensive interface circuits to assemble, store and transmitthe 48 samples of speech that are conveyed in each cell.

The CCR units of the access switches are connected to entities whichgather outgoing traffic and which distribute incoming traffic. TheATM-CM, to which a CCR is connected, switches individual cells of itsinput and output ATM streams between CCRs and Asynchronous Transfer ModeInterface Units (ATMUs), described further below. This unit distributesintra-cell traffic to a plurality of switch modules (SMs) of a 5ESSswitch or distributes such traffic to one or more stand-alone switches.It may be desirable in the future to connect switch modules of more thanone 5ESS switch to an ATMU.

Much of the traffic carried through the ATM signaltransmission/crossconnect network 10 is CBR traffic wherein theindividual CBR cells in each 125 μs frame are switched to a destinationCCR unit. A CCR unit is used advantageously for switching toll voicetraffic, because the access to a transit crossconnect for switching ATMcells provides the CCR with the ability to access a large number ofdifferent destination CCRs with different cells. The provision ofpermanent virtual paths (PVPs) to these destination CCRs and therelatively low rate of activation and deactivation of these PVPs allowsfor a relatively low rate of changes of paths in the network 10. Therouting pattern for a particular permanent virtual path (PVP) does notchange as long as the PVP is provisioned; the CBR cells can be routedaccording to a PVP as long as that PVP remains active. Similarly, PVPsin the ATMU also have a relatively long life. The dynamic portion of theswitching of the ATM units in the access switches is primarilyassociated with the switching of VBR cells whose headers may bedifferent with each 125 μs frame and which must be switched accordingly.The PVPs through the ATM-CM also have a relatively long active lifesince each PVP represents traffic from an ATMU to a source CCR, i.e., toa particular transit crossconnect. The configuration of the internalATMU mapping data and the CCR internal remapping data change morerapidly as individual PCM voice call connections are established anddisconnected. These units map individual PCM streams into specificlocations within a PVP composite cell.

FIG. 6 shows the exchange of messages required to complete the selectionof a PVP and a PVC within that PVP. The ingress access switch signals tothe egress switch (message 3) the identity of the source and destinationparties, and the identification of the PVP and the PVC of network 10.The destination node returns with an acknowledgment (message 4) properlyconfirming that path.

The access switches are connected to the central SONET/ATM signaltransmission/crossconnect network 10 by SONET/ATM access links 6. Theterm SONET (Synchronous Optical Network) is used herein to refer toeither or both of the U.S. standard (SONET) or the European standard SDH(Synchronous Digital Hierarchy). SONET/ATM means SONET or SDH signalsused to transport an ATM signal stream carrying ATM cells. Although notshown in FIG. 6, but shown in the details of block 10, FIG. 11, it isalso possible to have links directly connecting the access switcheswithin the network for those cases in which there is a large volume oftraffic between two access switches. In this directly connected case,that part of the network functions as a 2-stage network (see FIG. 2),and a CCR function is only required in one of the two access switches orin one direction (e.g., incoming) in each of the two access switches.

The access switches themselves are accessed by a plurality of localswitches. As indicated in FIG. 7, the local switches are connected tothe access switches through digital facilities, such as the U.S. 24channel conventional T carrier facilities or the European 32 channelsystems for carrying PCM signals, which signals are converted in theaccess switches to CBR cells of ATM signals. The CBR cells discussedherein are primarily composite cells, each cell carrying data for aplurality of communications.

FIG. 8 illustrates one 125 μs frame of a typical ATM signal that appearsat the output of an Asynchronous Transfer Mode Interface Unit (ATMU)(FIG. 24). A 125 μs frame consists of a number of CBR cells and a numberof VBR cells. For convenience, these are shown as being grouped at thebeginning and end of each frame, but it is also possible to intersperseVBR cells among groups of CBR cells. The advantage of grouping the CBRcells in the indicated manner is that priority of CBR cells can beassured and the design of the cell list processor (FIG. 26, block 630)is simplified. Signals coming into an ATMU are interspersed CBR and VBRcells. CBR cells are transmitted from an ATM-CM (block 550, FIG. 23) assoon after they are received as possible, thus giving them priority overVBR cells; the output of an ATM-CM connected to an ATMU therefore ismore likely to have CBR and VBR cells interspersed.

FIG. 9 illustrates the content of a composite CBR cell and a VBR cell ofa type used, for example, for transmission of signals across the ATMtransmission/crossconnect network 10. The content of a CBR cell includessignals for a plurality of channels. Since an ATM cell comprises a 5byte header and a 48 byte payload, one attractive arrangement is to havethe 5 byte header identify the particular virtual circuit represented bythe CBR cell, and to have the CBR cell payload contain the individualbytes (PCM samples) of 48 voice channels (64 kilobit PCM single channelDS0 signals). That is the format of a CBR cell between CCR units.

In the preferred embodiment, a CBR cell as generated in the ATMU andtransmitted between the ATMU and the ATM-CM carries PCM samples plus asupervisory signaling bit for only 42 separate channels in the 48-bytepayload of an ATM cell. The transport of this supervisory bit, named theE-bit herein in conformance with 5ESS switch terminology, allows thesupervisory state to be carried across module boundaries in the accessswitch, thereby simplifying compatibility with the present 5ESS switcharchitectural plan and software.

Described herein are arrangements for transporting a PCM sample plus onebit (the E-bit) having a specific purpose (supervision). More generally,A bits can be transported internally in a switch using the sameprinciples, namely, transporting 8+A bit segments for each conversation,the A bits conveying information for an arbitrary function; of course,the greater the value of A, the fewer segments can be transported ineach cell.

The VBR cell illustrated in FIG. 9 comprises a header and a payload,wherein the payload is associated with a single channel and a singledestination. In effect, a VBR cell represents part of a packet of databeing transmitted from a source access switch to a destination accessswitch of the toll network.

It is, of course, also possible to have a CBR cell all of whose payloadis devoted to a single communication, if the communication is acommunication such as the 1.5 megabit/sec. signal required for acompressed television signal. A similar type of CBR cell, operating at amuch lower repetition rate, can also be used for transmitting packetizedvoice signals, wherein 48 samples of a single voice channel aretransmitted in one cell. For broadband signals, such as High DefinitionTelevision (HDTV) signals, it is more convenient to connect thesesignals directly to the ATM-CM. Based on the use selected for the CBRPVP as selected by the originating access switch, the payload in eachcell is used as selected, with the same disposition being made for allcells transmitted over the CBR PVP for the duration of the existence ofthat PVP.

FIG. 10 illustrates one CBR cell carrying two-way traffic, the first nbytes carry channels 1 to n of outgoing traffic, and bytes (n plus 1 )to 48, carry (48 minus n) channels of incoming traffic. As discussedfurther hereinafter, the permanent virtual paths of this preferredembodiment are two-way, that is they consist of two-paired oppositelydirected uni-directional PVPs. The two-way traffic described herein isthe composite cell traffic that is originated from the two ends of aconnection between a source and a destination. The object of the layoutof time slots in a cell, such as that shown in FIG. 10 is to allow eachdirection of traffic to seize a time slot within a composite cellwithout encountering the concern that the other end is seizing the sametime slot at the same time ("glare"). Since an egress CCR assigns achannel, the bytes for outgoing traffic are seized by one CCR, those forincoming traffic by the other CCR. This avoids "glare" problems whereina channel is seized simultaneously for different cells from the two endsand where some back-out procedure is required. Since assignment of idlechannels is upward from 1 for outgoing traffic and downward from 48 forincoming traffic, if many fewer than all of the channels are active, itwill generally be possible to move the dividing point, set in this casebetween channel n and channel n plus 1, in the direction of additionalrequests for channels. The two-way traffic CBR cells are particularlyuseful for carrying traffic between a source and a destination CCR whenthere is relatively little such traffic being offered. One-way trafficCBR cells are useful when there is substantial traffic, because seizureis always from one end and "glare" problems can be avoided naturally.

Because of the large number of switches that are required in, forexample, the AT&T toll network, it is desirable to provide for anarrangement to switch cells from a plurality of CCRs in a transit ATMcrossconnect 600 (FIG. 11 ). The transit ATM crossconnect is a switchwhich receives CCR inputs from a plurality of access switches andswitches cells from these inputs for transmission over an ATM facilityto CCRs of the plurality of access switches. As a result, in general, aCCR would have in its output individual CBR cells each of which carriessignals destined for a single CCR, but the stream may have cellsdestined for a plurality of CCRs, the plurality being the CCRs to whichthe transit ATM crossconnect is connected. The CCRs connected to thetransit crossconnects are full duplex CCRs for switching timeslots amongcells in both the incoming and outgoing ATM data streams. Each such CCRis terminated not on another CCR but on a unit, the transitcrossconnect, which switches cells to different ATM streams thusgenerating totally different ATM streams in contrast to the directlyconnected CCRs which bypass a transit crossconnect. For a CCR that isconnected to only a single CCR, one of the CCRs, or one direction ofeach CCR can be simplified.

The transit crossconnects 600 have the function of switching cells froma plurality of ATM input data streams to a plurality of ATM outputstreams. Thus, the primary function is the same as the ATM-CM. Onecandidate for the transit crossconnect, therefore, is an ATM-CM. Asecond possibility is an ATM digital access and crossconnect system(ATM-DACS) which system is used primarily not as an active switch but asa means for providing generally long term connections among ATMfacilities. The choice between an ATM-DACS and a unit like the ATM-CMdepends on the rate at which new connections must be established and oldconnections removed in order to serve dynamic traffic adequately; theATM-CM generally must establish virtual circuits more rapidly in orderto accommodate packetized data transmission. At this time, it isbelieved that an ATM-DACS should be able to serve as an transit ATMcrossconnect. The ATM-CM can also perform the function of providingconventional permanent crossconnects. It can provide crossconnectcapability for ATM streams entering an access switch. Moreover, forexample, spare capacity of an ATM-CM for a first access switch can beused for providing crossconnects between a plurality of other accessswitches, i.e., it can provide the functionality of one of the blocks600 of FIG. 11. In addition, as discussed with respect to FIG. 4, twocrossconnects may be connected through a CCR 4000. Such a CCR mayrequire more dynamic control than the transit ATM crossconnects shown inFIG. 11. Further, to create greater access for access switches whichhave only a few cells destined for a particular destination accessswitch, an ATM inter-crossconnect link 7 may be provided to connect twocrossconnects without performing cell remapping or to return a remappedcell to an ATM crossconnect; such connections also provide greaterreliability in case of ATM link failures. The blocks of FIG. 11 arecontrolled by a centralized network control system (not shown) of a typeused for controlling crossconnect systems.

FIG. 12, discussed in the Overview, is a diagram showing therelationships among the various blocks connected to an ATM network. Inthe exemplary embodiment, the inputs are switching modules which areconnected to T-carrier facilities as well as analog trunks and analog ordigital lines, and subscriber loop carder systems. The outputs of theswitching modules are 5ESS switch Network Control and Timing (NCT)links, each comprising PCM samples for a multiplicity of telephoneconversations. Each NCT link is connected to one ATMU. The plurality ofNCT links from each SM are connected to one or more ATMUs. In the ATMU,the PCM samples are packaged into cells of a plurality of ATM outputs.The ATM outputs of an ATMU are connected to one ATM Communication Module(ATM-CM) which keeps cells intact. The signals within each cell,destined for a single CCR, thence a single transit crossconnect, haveaccess to a large number of access switches of a telecommunicationsnetwork.

The output of the ATM-CM is connected to a group of CCRs 4000. Each CCRis controlled by ATM management module (AMM) 1202 (FIG. 30) forrecording and controlling the virtual connections established within thecontrolled CCR. Each of the CCRs 4000 in an access switch 1 (FIG. 11 )performs a cell remapping function, for switching any channel on anycell on its input ATM bit stream to any channel of any cell on its ATMoutput stream. The CCRs 4000 are part of access switches 1 connected toa SONET/ATM signal transmission/crossconnect network 10, shown in FIG.11, by access links 6 which, in the preferred embodiment, also use SONETto carry the ATM signals. This network switches and transmits the CCRinput/output links 6 of the access switches 1, directly interconnectingsome, interconnecting others through Transit ATM crossconnects 600.

It may be desirable to provision some virtual circuits as "2-1ink"virtual circuits, each "link" representing a complete path between twoaccess switches. The Real Time Network Routing arrangement, used in theAT&T toll network and described in U.S. Pat. No. 5,101,451, issued Mar.31, 1992, can be used for selecting an optimum (i.e., relatively lessheavily loaded) "2-1ink" circuit out of a plurality of such circuits.This is especially true where the expected CBR voice traffic between twoaccess switches is much less than one cell per time frame, (i.e., lessthan 48 channels). The "two link" circuit shown in FIG. 21 requirestraversing two CCRs and an ATM-CM at the intermediate node, and isequivalent to a five stage connection in the terminology of FIGS. 1-4.An alternative for providing an alternate route is a "four-stage" switchas discussed with respect to FIG. 4, using a CCR in network 10 as shownin FIG. 11.

In the exemplary embodiment, the ATM signals between the ATMU and theATM-CM carry 9-bit segments for each conversation, an 8-bit PCM sampleand the E-bit for that channel. Inputs from an ATMU to an ATM-CM may beswitched back to the same or a different ATMU, or be switched to a CCR.Within the ATM-CM, individual cells of an ATM stream are switched to oneof a plurality of ATM streams, but the content of each cell is keptintact. Since PCM signals that are switched back to the ATMU or from oneATMU to another require, in the preferred embodiment, the maintenance ofthe E-bit, the ATM signals passing through the ATM-CM maintain the E-bitsignal associated with each PCM sample, including those signals passedfrom the ATM-CM to the CCR. In the preferred embodiment, each ATMU isconnected to only one ATM-CM; this simplifies the provisioning ofpermanent virtual circuits. Alternative arrangements wherein an ATMU isconnected to two or more ATM-CMs for reliability or for greater trafficflexibility are also possible.

When the digital facilities themselves carry packetized data, then thispacketized data is processed by a Packet Switch Unit 519 (FIG. 23)within a switching module (SM) 510 of a digital switch, sent via theTime Slot Interchange Unit (TSIU) 517 of that SM to the ATMU 540 whereit is converted to VBR ATM cells and transmitted over VBR PVPs to an ATMCommunication Module (ATM-CM) 550. The ATM-CM has as its inputs andoutputs a plurality of ATM bit streams, and has the capability ofswitching individual ATM cells from one of these bit streams to another;unlike the CCR, the ATM-CM does not unpack data within the payload (48bytes) of an ATM cell, but only switches complete cells. In addition,signaling channels are treated as CBR or VBR channels and aretransported, accordingly, in CBR cell channels or single channel VBRcells of the type described below. Significantly, by transportingsignaling channels through the ATM network, the necessity for a separatesignaling network, using Signal Transfer Points (STP), is avoided. Forexample, signaling information can be sent from a source switchingmodule to a destination switching module over the physical paths shownin FIG. 20.

Thus, the SM is arranged so that any input of an incoming PCM stream maybe placed in any time slot of any output NCT link. The ATMU is arrangedso that any time slot on any NCT link input may be placed in any slot ofany cell of any ATM output of the ATMU. Each ATMU is connected to asingle ATM-CM in the preferred embodiment. The ATM-CM can switch anycell on any input to any cell on any output, but cannot switch datainternal to cells. The CCR is arranged so that any channel (slot) of anycell of the input line may be connected to any channel of any cell ofthe output line. The SM, ATMU and CCR control memories are updated oneach new call. In contrast, the ATM-CM has the function of connectingany cell of any input to any cell of any output line. Consequently, thecontrol memory contents of the ATM-CM are relatively static for thecomposite cell CBR PVPs and are changed only when it is necessary toactivate a transmission path (which transmission path can then serve 42or 48 calls). It is therefore desirable to provide an ATM-CM controlmechanism with a repertory of permanent virtual circuits which may beactivated as needed.

Those outputs of an ATM-CM which are connected to the CCR do still carry9 bits for each voice channel. In the preferred embodiment, each CCR hasone ATM input connection and one ATM output connection, (treating theconnection to the ATM-CM as the input and the connection to theSONET/ATM signal transmission network 10 or to the same or anotherATM-CM as the output). In the CCR, only time division switching occurs.The contents of each composite cell are disassembled and reassembled innew time positions within a plurality of destination cells of the CCR.In the CCR, any PCM input sample in any position of any cell, may beswitched to any position of any cell of the output of the CCR. Further,since the outputs of the CCR go to the ATM transmission network, thereis no further need to carry along the E-bit associated with each PCMsample.

Note that while in the preferred embodiment, the CCR has only onephysical ATM input stream and one physical ATM output stream, futurechanges in technology may make it desirable to provide, effectively, twostages of space division switching, one in the ATM-CM and one in theCCR, by providing the CCR with several input and output data streams.Similarly, and especially in view of the limitation on capacity imposedon the ATMU by the speed of its signal memory 620 (FIG. 24), it may bedesirable to provide the ATMU with only a single ATM output stream. Thepresent configuration assumes that the ATM-CM can switch cells from anyinput to any output, and each ATMU is connected to only a single ATM-CM;therefore, multiple ATM output streams from the ATMU do not increase the"reach" of the ATMU, provided the single output has as many cells perframe as a plurality of ATM outputs from the ATMU.

For an intra-access switch call (a call which does not involve the useof the CCR) a control mechanism which is either a switching moduleprocessor or an administrative module for the switch selects first ahalf-path between the originating switching module and the ATM-CM andthen establishes the other half-path between the ATM-CM and theterminating switching module. Usually, the path through the ATM-CM willbe an activated pre-provisioned virtual path known to the administrativemodule and provided to whichever processor(s) are used for controllingthe path establishment in the SM and the ATMU; occasionally a newpre-provisioned virtual path must be activated, deactivated, or twoactivated paths consolidated.

An interswitch call via a transit crossconnect, requires the use of aCCR for each half-path. The half-path for the originating end isprovided with the identification of a circuit group cell in the ATM-CMthat connects the ATMU with the selected CCR and this half-path isestablished in essentially the same way that a half-path for anintraswitch call is established. In the CCR, it is necessary only tofind an available channel in any cell of the ATM output which isdestined for the destination CCR. The destination CCR must be informedof the identity of the cell and channel within the cell that is carryingthe particular conversation and the same type of path establishmentprocedure is then carried out at the terminating access switch.

For an inter-switch call, each access switch half path is between aswitching module and a CCR, and the processor of the module (a switchingmodule processor) and the processor of the CCR (Administration Module530, FIG. 23) control the termination of the two ends of the half path.For an intra-switch call, the two ends of the path are terminated at theswitching modules, and the switch module processor (SMP) of thesemodules control the termination of the path.

Since the signals interconnecting the SMs, ATMUs, ATM-CM, CCRs andtransit crossconnects are all signals capable of being transmitted overoptic fiber, the units need not be in the same location. For example, anATM-CM, thence a CCR may gather traffic from a group of SMs that arescattered over a relatively wide area. While a CCR could be colocatedwith a transit crossconnect instead of being colocated with ATM-CM, thearrangement in the preferred embodiment has the advantage of locatingthe CCR near its source of control information, i.e., the controls ofthe ATMU, ATM-CM and CCR. The object of this type of networkconfiguration is to minimize the number of cases in which furtherreswitching is required. For cases in which there is relatively littletraffic between two regions, a single ATM facility to a single transitcrossconnect such as the top unit 600 in FIG. 11 (or two geographicallydiverse ATM facilities to two different transit crossconnects to providebackup) may be used to interconnect all traffic that has been gatheredfrom a large region and transmitted to another crossconnect such as thebottom unit 600 in FIG. 11 for connection to the large region at theother end for subsequent redistribution.

In this network, the ATMU can arrange different PCM samples intodifferent cells and, since the ATMU has a plurality of ATM outputs, canalso perform space division switching to place any particular cell onany of the outputs. (The latter facility is not really needed since theATM-CM can switch any input to any output.) The ATM-CM performsswitching of cells from any of its input ATM streams to any of itsoutput ATM streams, but without changing the payload contents ofindividual cells, i.e., PCM samples am not switched from one cell toanother. The CCR can move PCM samples between different cells but inthis embodiment performs no space division switching since it has onlyone physical ATM input stream and one physical ATM output stream. Thetransit crossconnect performs a switching function similar to that ofthe ATM-CM, namely, any input cell can be moved to any output ATMstream, but PCM samples are not moved from cell to cell. Consequently,individual cell outputs of the ATMU are such that any given cell has asits destination a single CCR which is connected to a single transitcrossconnect; therefore, all PCM samples in a cell generated by the ATMUhave as a common destination a given transit crossconnect. (This, orcourse, refers only to interaccess switch cells since cells which remainin the same access switch are restricted only to having the switchingmodules or systems connected to a single ATMU as their commondestination. Similarly, those cells which are switched by the ATM-CM toa CCR that is connected directly to another access switch withoutpassing through a transit crossconnect, have, as their commondestination, that access switch.) The cells at the output of a CCR,which output is also an input to a transit crossconnect, have as theircommon destination a single destination CCR. The output of thedestination CCR is cells each containing PCM samples of switches ormodules connected to a single ATMU.

The topology of the switching plan can also be understood by examiningthe contents of different types of ATM cells. FIG. 13 shows thetransport of DS0's using the three types of CBR composite cells. Thetimeslots between SMs of one access switch are transported using singledestination composite cells (SDCCs). Timeslots destined for other accessswitches are transported internally within an access switch via multipledestination composite cells (MDCCs). In the particular embodiment usinga 5ESS switch, only 42 PCM samples (time slots) are transmitted in eachSDCC or MDCC. The other 6 bytes are used to transmit an E-bit for eachchannel (representing the supervisory state of that channel) and totransmit a Cyclic Redundancy Check (CRC) code. For SDCC cellstransmitted toward another SM of the same 5ESS switch as thetransmitting SM, the E-bit is preserved and transmitted back. For MDCCcells leaving the access switch, the E-bits and CRC are stripped priorto generating a TDC (Tandem Destination Cell) cell comprising 48 PCMsamples.

FIG. 13 is a block diagram showing an access switch using an ATM-CM forproviding connectivity, between 5ESS Switch Switching Modules; in thiscase the switching modules are used for providing connectivity to otherlocal switches. The ATM-CM provides access via the CCRs and network 10to other access switches. When the ATM-CM acts as a crossconnect,separate segregated ATM facilities can be connected to other localswitches from the ATM-CM. The SM in FIG. 13 is either a Switching Moduleof a 5ESS switch or a Switching Module-2000, described in Document015-372-101, "5ESS-2000", July 1993, available from the CustomerInformation Center of AT&T, but both will be referred to herein as anSM. The ATM Unit (ATMU) provides an interface between a 5ESS Switch SMand the ATM-CM. The SM interface to the ATMU is an NCT link carrying asequence of independent, single byte, 64 kbps time slots, as describedin the following section, each byte enhanced by eight additional controlbits including the E-bit previously described. The MDCC, SDCC, andtandem destination cell (TDC) signals shown in FIG. 13 are describedfurther with respect to FIGS. 14-17. FIG. 13 also shows a composite cellremap unit (CCR) for forming CCR output composite cells each of whichhas as a destination a single CCR 4000 (FIG. 42), or a single accessswitch 1 connected to such a CCR, but each such CCR output compositecell may carry channels that go to different ATMUs of a destinationaccess switch. In order to handle the special problems of the 5ESSswitch, (transporting "E-bits" between switching modules), the singledestination composite cells (SDCC) differ from the more general singledestination composite cells (TDC) described with respect to FIGS. 16 and17.

Internal to the switch, composite ATM cells are always used, but ininterfacing to the network there is the option of using either compositeATM cells or conventional packet voice ATM cells. The payload incomposite cells consists of 64 kbps samples from up to 48 separatecalls, whereas a conventional packet voice ATM cell contains 48 samplesfrom a single call.

The three types of composite cells are:

Single Destination Composite Cell (SDCC),

Multiple Destination Composite Cell (MDCC),

Tandem Destination Cell (TDC).

ATMU to ATMU connectivity uses the SDCC format. Since there are arelatively small number of ATMUs in a switch there is generally a lot oftraffic between all ATMUs. Thus, paths carrying SDCC represent anefficient use of bandwidth.

FIG. 14 is a representation of the SDCC format. The cell destination isanother SM.

A maximum of 42 simultaneous calls (and their associated E-bits) (48byte payload) is accommodated. The ATMU maps the time slots which havethe same destination ATMU into the same cell.

FIG. 14 is an example of a mix of time slot numbers which go to the sameATMU. The time slots are not necessarily in any order within the cellsince time slot numbers change as calls are set up and tom down. Somebyte locations (i.e., time slots) in the cell may not contain anyinformation since the cell is not necessarily full, i.e, there may befewer than 48 voice or data connections to/from the destination ATMU.

Although within the switch, SDCCs make efficient use of bandwidth thesame is not true for cells going to other switches. For example, aparticular ATMU in a switch may have very little traffic going to anATMU in another switch and would still have to use a path capable ofcarrying 42 channels with the result of having many empty (unused) slotsin the cell. In order to communicate more efficiently from an ATMU tothe Composite Cell Remap (CCR) function, time slots which are destinedfor multiple switches accessible from a single CCR of the source accessswitch via a transit crossconnect, are packed by the ATMU into a singleMultiple Destination Composite Cell (MDCC). FIG. 15 shows the cellstructure. The MDCC also carries 42 voice channels instead of 48 becauseof the desirability within the ATMU of generating a single outputformat, set by the requirements of the SDCC.

The destination of these MDCC cells is a single CCR of the originatingaccess switch. The mixture of cells with ATMU, access switch, or CCRdestinations and the number of time slots going to a particular switchis a function of traffic conditions and will vary from ATMU to ATMU. TheMDCC implementation allows reduced bandwidth between the ATMUs and theCCR because sparsely filled composite cells to/from ATMUs in otheraccess switches are avoided by packing the time slots into an MDCC.

Paths carrying Tandem Destination Cells (TDC) are used to provideconnectivity between CCRs. These TDCs can use either a composite cellformat or conventional packet cell (48 samples of one conversation inone packet) format as shown in FIGS. 16 and 17.

For implementation of the composite cell format shown in FIG. 16 the CCRextracts the time slots from the MDCC and aggregates them with the timeslots extracted from all received MDCCs from all the ATMUs in the accessswitch to form composite single CCR destination ATM cells. Thesecomposite cells have a single access switch (i.e., CCR) destination asspecified in the header. Messaging between the SMs and the CCR isnecessary to correlate the DS0 position in the ATM cell with thedestination switch identity. Since no E-bits or CRC check are needed forinter switch connectivity, up to 48 simultaneous calls can be handled bythe composite TDC ATM cell even if the switches used are 5ESS switches.Some DS0 positions may be empty depending on traffic conditions.

The CCR can also be used to convert the composite cell structure usedinside the switch to "conventional packet voice" TDCs for transmissionbetween switches as shown in FIG. 17. In order to carry out thisfunction, the CCR accumulates 48 voice samples from each of the calls(active time slots) in the MDCCs, buffers them and then sends them outto the destination switch. The conventional cell TDC can also be usedfor packetized data.

ATMU

The primary purpose of the ATMU is the mapping of single byte PCM timeslots into 53 byte ATM cells as well as providing the inverse function.The output of the ATMU towards the ATM-CM is ATM cells and toward the SMit consists of single byte time slots.

FIG. 18 shows the mapping between NCT PCM timeslot data and an SDCC orMDCC. FIG. 18 shows that up to 42 8-bit PCM samples from 42 distinctcalls, plus their corresponding E-bits, plus a 6-bit CRC check fieldmake up an SDCC or MDCC composite cell. The choice of a 6-bit CRC fieldis clearly an implementation choice and a larger or smaller CRC, or noCRC, could be implemented, as could other functions such as framing orparity information.

CCR

It is very inefficient if each ATMU on one access switch requiresdedicated composite ATM cells (53 bytes each) to each ATMU in all theother access switches to which it is connected. This inefficiency getsgeometrically worse with increasing numbers of ATMUs in an accessswitch. It also requires an Operation, Administration, Maintenance andProvisioning (OAMP) update for every switch whenever an ATMU is added inany switch. In the preferred embodiment, ATMUs send ATM cells to theComposite Cell Remap (CCR) function in their access switch, to aggregatethe time slots from all ATMUs in that switch which have the same accessswitch destination into ATM cells and send them to the destinationaccess switch.

The primary functions of CCRs are:

1. An operation must be performed which "remaps" internal compositecells into external cells. This remapping is done by the CCR.

2. For a 5ESS switch, the CCRs also perform the E-bit functions forinter-switch narrowband-to-broadband calls, since only one SM isinvolved in these type of calls. Refer to the call scenarios of FIGS.34-41 for more detail regarding the E-bit functions of the CCRs.

The CCR takes DS0s that are destined for the same access switch (CCR)from possibly multiple MDCCs and places them into one or more TDCs thatare switched to the given destination access switch as shown in FIG. 19.

At the destination access switch, an inverse process occurs. In thiscase, DS0s from the TDCs are mapped by the CCR into MDCCs that areinternally switched by the ATM-CM to the proper terminating ATMU. Aspreviously discussed with respect to FIG. 11, CCRs are interconnectedprimarily via transit crossconnects.

The foregoing composite cell discussion outlined the mechanism by whichDS0s are transported between two ATM-CM offices. The signalingprocedures work in concert with the composite cell hardware to negotiateand set up composite cell ATM paths and composite cell DS0 locations.

As in the case of the SDCC cells discussed earlier, a plurality ofinactive virtual paths are initialized in the transit crossconnect. WithMDCCs or SDCCs, whenever no channels are available between a particularsource CCR and destination CCR or switch, an inactive virtual path isseized and made into an active virtual path. Channels are seized onactive virtual paths having an available channel. For MDCCs, whichconnect ATMUs and CCRs, the assignment of a channel is performed notwhen the virtual path is activated but when the particular communicationis requested. Any activated MDCC and TDC, interconnecting source anddestination, with at least one available channel can be assigned. ForSDCCs and MDCCs, new virtual paths are activated as needed on a per callbasis, and channels within the virtual paths are assigned on a per callbasis.

FIG. 20 shows functional block diagram of a call between an originatingand terminating access switch, showing signaling and virtual paths andchannels.

In the figure, the following functional entities and associations areindicated:

SM: terminates N-ISUP (Narrow Band ISUP) signaling and T1 from LocalExchange Carder (LEC) switch, and provides mapping between LEC DS0 and atimeslot to the ATMU,

ATMU: provides mapping between SM timeslots and SDCC/MDCC ATM paths andDS0 byte location in the selected cell of the virtual path.

ATM-CM: provides mapping from its incoming port, Virtual Path Identifier(VPI), and Virtual Circuit Identifier (VCI) to its outgoing port, VPI,and VCI. The VPI and VCI refers to standard ATM terminology, foridentifying a virtual path in the terminology of this application. VPIand VCI are transmitted in the header of an ATM cell.

CCR: provides mapping between MDCC and TDC ATM paths and DS0 bytelocations.

BB-CP: establishes TDC ATM path and DS0 byte location between two ATM-CMoffices. The Initial Address Message (IAM) message carried on theSignaling System (Number) 7 (SS7) signaling virtual path contains theVPI, VCI and byte location in the cell.

Since composite cells are used inter-switch, an additional parametermust be included in the signaling message to the far-end ATM-CM office.This additional parameter identifies the composite cell byte position ofthe DS0 associated with the call. This additional parameter isidentified as CCB (Composite Cell Byte) in the Initial Address Messageshown in FIG. 20 above.

Permanent virtual circuits are used in the portions of the path whereinonly cells are switched, i.e., the ATM-CM in the source and destinationaccess switches and the transit crossconnect. (The term "virtualcircuit" as used herein refers to a single channel and is thusequivalent to a "virtual channel".) Individual virtual paths areestablished in the ATMU connected to the source and the destinationstation, and in the CCR of the source and destination access switch. Foran intra-access switch call, no CCR and no transit crossconnect isinvolved so that the only permanent virtual paths are those into and outof the ATM-CM. Thus, when there is a request to establish a path, thesource access switch finds a permanent virtual paths interconnecting theappropriate ATMU and CCR and connects the source PCM channel through theATMU to the selected CCR in the ATM-CM of the source access switch anddirects the CCR to make the appropriate connection of that channel asreceived from the ATM-CM to a cell of the PVP through the transitcrossconnect. Similar actions are performed in the destination accessswitch. For an intra-access switch call, a PVP in the ATM-CM of theaccess switch is selected which interconnects the ATMU connected to thesource PCM channel and the ATMU connected to the destination PCMchannel. These two ATMUs (which may be the same ATMU) are then directedto establish a connection between the source or destination PCM channeland the selected PVP of the ATM-CM.

The permanent virtual circuits in this preferred embodiment are two-waycircuits. Since the ATM signals themselves are uni-directional, thismeans that a pair of PVPs is assigned for a specific communication or,in the case of composite cells, for a group of communications. When aparticular communication is assigned to a particular PVP carryingcomposite cells, it is assigned to corresponding positions within thecell for the two uni-directional PVPs that compose the bidirectionalPVP. For the case in which a packetized data communication is beingestablished, where data communication may be heavily or entirelyuni-directional, this simply means that the ATM switches andcrossconnects will send fewer cells for that communication in onedirection than in the other. The activation of a PVP merely requiresthat the active memory of the switch or crossconnect contains that PVPbut does not in itself require the use of additional resources, theadditional resources being utilized only when cells for that PVP areactually being transmitted.

FIG. 21 shows a signaling network overview (omitting the ATMUs forsimplicity) of an ATM network using the ATM-CM units. In this networkarchitecture, the Local Exchange Carrier (LEC) switches communicate withthe access switch using Narrowband ISUP (N-ISUP) signaling. The DS0sterminate on narrowband SMs in the originating access switch. Asdescribed previously, the originating access switch converts the DS0samples into composite cells which are transported to the far-end accessswitch, via direct interfaces on the ATM-CM. As shown in FIG. 21,alternate routes can be established via the CCRs and ATM-CM of otherdesignated access switches. A simpler, less expensive alternate route isestablished using an intermediate CCR as shown in FIG. 11. In all cases,only 2 narrowband SMs are involved in the call.

2.2 Architecture Overview

Shown in FIG. 22 is the architecture for a fully integratednarrowband/broadband access switch.

Noteworthy aspects of this architecture are:

Broadband network and user interfaces terminate on the ATM-CM.

Remote ATM Switching Units (RASUs) can be supported off the host ATM-CM.

The full complement of existing narrowband interfaces, signalingprotocols, and features are supported by the SMs.

The broadband components can be common with other applications.

The ATMUs are located in the ATM-CM.

Composite cells are used internally within the switch, between ATMUs andCCRs for incoming/outgoing narrowband-to-broadband calls, as well asbetween ATMUs for intra-switch narrowband calls).

Broadband ISUP is the signaling protocol used between switches.

E-bits are transported over the NCT links, and mapped into the compositecell segment by the ATMU. For SM-to-SM calls, this results in no changeto the E-bit processing in the SMs. For narrowband-to-broadband calls,E-bits are transported in the composite cells between the ATMU and theCCR. E-bit processing is then perforated by the CCR. The CCR removesE-bits for outgoing calls. Further, E-bit processing in the CCR mimicsthat which would be done by an SM, since the call processing software inthe SM is not affected by the introduction of the ATM-CM. This meansthat it should be transparent to the one SM in the connection that thereis not another SM (i.e., the CCR manipulates the E-bit just as an SMwould). Thus, when a connection drops, the CCR turns off the E-bit,which thereby allows the SM to recognize the loss of continuity to thefar end. No E-bits are transported in the cells between CCRs.

The functional distribution among the components is as follows:

The AM provides support (pumping, initialization control, etc.) for theATM-CM, the SMs, the ATM Packet Handler (APH), and the CCRs in theswitch. In addition, the AM performs call processing functions similarto a standalone narrowband switch, such as trunk hunting and NCTtimeslot selection. The AM provides information to the ATM-CM necessaryfor per-call path setup when a narrowband termination is involved in thecall.

For this preferred embodiment, the AM is required to be a relativelyhigh speed processor such as AT&T's 3B21D in order to support theFuturebus+ extension. This permits the connection of a BroadbandSignaling Processor (BB-SP), a Broadband Control Processor (BB-CP),Broadband OAMP System (BB-AP) and Broadband Terminal Servers to beattached directly to the AM and the ATM-CM.

The BB-CP provides the broadband call processing and subscriber servicesfor the switch. It terminates layer 3 B-ISDN (Q.93B) and B-ISUP(Broadband ISUP) (Q.764) signaling, and maintains the subscriber profilefor the broadband subscribers on the system. Broadband call processingsoftware in the BB-CP interworks with the narrowband call processing andfeatures on the SMs.

The BB-SP is the broadband signaling processor and performs thetermination of the ATM and ATM Adaptation Layer (AAL) layers, for boththe Network-Node Interface (NNI) and the User-Network Interface (UNI).

The BB-AP handles all OAMP activities related to the broadbandcomponents of the switch. This includes integrity and recovery of theBB-CP, BB-SP, and Remote ASUs.

The Terminal Server supports asynchronous terminals and interface accessunits for broadband Operations Support Systems (OSSs).

The ATM-CM is the switching fabric for switching cells between differentATM input and output bit streams. The ATM-CM terminates the lowestlayers of the UNI and NNI interfaces. It is also responsible for controlof the per-call and provisioned Virtual Path (VP) connections and allbandwidth allocation. The ATM-CM is responsible for the integrity andrecovery of the ATMUs.

The CCR remaps internal composite cells to standard external ATM cells.The external cell to composite cell/byte mapping tables in the CCR aremaintained by the ATM-CM. It is also possible to carry out the functionin units associated with the CCR echo canceling.

The ATMU, as described earlier, provides conversion between NCTtimeslots and ATM composite cells. The timeslot to composite cell/bytemapping tables in the ATMU are maintained by the ATM-CM.

The RASU (Remote ATM Switching Unit) is an optional unit comprised of anASU-2000 fabric, of the type used in the ATM-CM remoted over an ATMfacility.

The SM-2000 or other SM provide all the narrowband call control,supplementary services and features, as described earlier in thisdocument. Call processing in the SMs interworks with broadband callcontrol in the BB-CP for connections between narrowband and broadbandsubscribers.

For International applications, the GSM-2000 (a global version of SM2000) directly terminates the lower layers (i.e., Message Transfer Pan(MTP)) of the narrowband Signaling System No. 7 inter-office signalingprotocols. For the AT&T network switches, a CNI (Common NetworkInterface) Ring performs this function.

The APH is an optional unit which is present if an RASU is equipped inthe system. The APH performs SDH termination and ATM Adaptation (AAL)layers functions, and provides a communication mechanism for accessingunits in the system which are remoted over ATM, such as the RASU. In thepreferred embodiment, ATMUs and CCRs receive control information overcontrol links 541 and 551, respectively (see FIG. 18) from the callcontrol units (Administration Module, Broadband Call Control, BroadbandSignaling, Broadband OAMP, etc.) This is in contrast to the SM whichreceives control messages over control time slots of the NCT linksconnecting these units to a message switch.

DETAILED DESCRIPTION

FIG. 23 is a block diagram of an access switch 1. The 5ESS®switch,manufactured by AT&T, and extensively described in AT&T TechnicalJournal, Vol. 64, No. 6, Part 2, July-August 1985, pages 1303-1564, isthe switch described for use with applicants' invention. It includes aplurality of switching modules. The input from the local switches 2(FIG. 6) are terminated on switching module 510. This switching modulecomprises both circuit and packet switching units, such a module isdescribed in M. W. Beckner, J. A. Davis, E. J. Gausmann, T. L. Hiller,P. D. Olson and G. A. VanDine: "Integrated Packet Switching and CircuitSwitching System", U.S. Pat. No. 4,592,048. This module is controlled bya switching module processor 511 which communicates with a messagehandler 513 for receiving and transmitting messages. The T-carrierinputs from local switches 2 are terminated on digital interface 515 andare switched by time slot interchange 517. Since the signals arriving atthe digital interface also contain packet switched signals (for example,signals from the D-channels of Integrated Services Digital Network(ISDN) sources) a packet switching unit 519 is also provided. Theoutputs of this packet switching unit are sent to the time slotinterchange unit for further switching onto output digital links of theSM 510. In addition, a SONET interface unit 521 is provided forinterfacing with PCM signals carried over SONET facilities from thelocal switches. The outputs of the switching module 510 are a group ofnetwork control and timing links (NCT 523, . . . ,524) (typically, up to20). The NCT link signals are carded over optic fiber links and canreadily be made long enough to allow a SM to be remotely located. Asubgroup of these 20 links is then terminated in an AsynchronousTransfer Mode interface Unit, (ATMU) 540 which can also terminate NCTlinks from other SMs. Other subgroups from this SM can also be connectedto other ATMUs, the latter also being connected to ATM-CM 550. Theoutput of the ATMU is a plurality of SONET/ATM signals to the commonbroadband platform (ATM-CM) 550. The AMM, an extension of theadministrative module (AM) 530, is used to control switching connectionsin the ATM-CM 550 and to carry out common functions for a plurality ofswitching modules connected to a particular ATM-CM 550. The ATM-CM isalso used to switch signals between different switching modules 510connected to ATM-CM 550 in order to handle tandem calls between localswitches 2 that are not connected to a common switch module.

FIG. 23 shows a configuration wherein the ATMU can be separatedphysically from both the ATM-CM and the SM; both the NCT link and theSONET/ATM link are arranged to transmit signals over longer distances.Clearly, if the ATMU abuts or is part of either the SM or the ATM-CM,these facilities can be simplified.

As an ATM crossconnect unit, the ATM-CM is able to perform the functionof connecting ATM cells between ATM inlets and ATM outlets. To ensurethat the composite CBR cells that carry voice traffic are not delayed orlost, the CBR cells are given high priority. They are sent on facilitiesthat are selected to have sufficient bandwidth to support theirtransport, and buffering is always able to accommodate these cells.Simulations have demonstrated that the probability of a queueing delayacross an ATM-CM in excess of 50 μs for a CBR cell, even when facilitiesare fully loaded, is less that 1×10⁻¹¹. Narrowband VBR signaling andother priority cells are guaranteed transport via margins of bandwidthin the facilities that are reserved for this purpose. Those VBR cellsuse buffers that are separate from the CBR cells, even though they areon the same facility. Broadband connections use separate facilitiescoming directly into the ATM-CM. These broadband signals use differentbuffers in the ATM-CM that are separate from the narrowband CBR and VBRbuffers.

The ATM-CM is connectable via CCRs to a transit network 10, and is alsoused to interconnect a group of ATMUs and their connected SMs to form asingle giant switching system or switching system cluster, the giantsystem or cluster being connected to other switching systems via anetwork connected to the SMs. During a transitional period, the existingtime multiplexed switch (described in the AT&T Technical Journalreference, for example, on pages 1425-1426) can continue to carry partof the inter-SM traffic and the ATMUs and ATM-CM can carry the rest.

While in this specific embodiment, the inputs to ATMUs are from a groupof switching modules of a single switching system, such as the modulesof a 5ESS switch, the teachings of applicants' invention are equallyapplicable if separate switching systems, instead of switching modules,are connected to ATMUs.

The block diagram of FIG. 23 is a specific embodiment of the inventionbased upon the 5ESS switch architecture. The Asynchronous Transfer ModeInterface Unit (ATMU) provides the interface between a switching moduleand the ATM-CM. The ATM-CM acts as a combination communication modulefor interconnecting connected switching modules and as an ATMcrossconnect for connection via ATM links to other ATM-CMs. The 5ESSSwitch Module normal format for voice and data transport is a singlebyte time slot. The ATM-CM format is a 53 byte cell containing 42 9-bitsegments, each segment containing an 8-bit PCM sample and one E-bit, forvoice and data transport, and 5 bytes for overhead. The ATMU providesthe conversion between the single segment time slots and the 53 byte ATMcells. It also provides arrangements for transmitting the multiple timeslots required, for example, for a 384 kilobit/second channel, intodifferent segments of a single ATM cell. The 5ESS Switch Moduleinterfaces to the ATMU over Network Control and Timing (type 1 or type2) (NCT) links and the ATMU interfaces to the ATM-CM over SynchronousDigital Hierarchy (SDH) or its U.S. version, Synchronous Optical Network(SONET), links. The SDH or SONET links transport ATM cells. If, as inthe preferred embodiment, the ATMU is part of a physical unit containingthe ATM-CM, there is no need to provide either SDH or SONETfunctionality; in this case, the ATMU interfaces with the ATM-CM via ATMcells. By providing voice and data switching the ATM-CM eliminates theneed for the time multiplexed switch used in the communications moduleof the prior art 5ESS system. In this preferred embodiment, the switchmodule (SM) is an SM-2000, previously referred to, larger than thatdescribed in the AT&T Technical Journal reference to take advantage ofthe higher speed circuitry available today. The dimensions of the SM arediscussed below, as appropriate. While in this embodiment the ATM-CMinterfaces via the ATMU with switching modules of one switching system,it can equally readily interface with one or more stand-alone switchingsystems.

Although not currently planned for the preferred embodiment, the ATM-CMalso could provide SM message routing to both the Administration Module(AM) (of the 5ESS switch), and to other SMs, thus eliminating the needfor intra-switch message router. The ATMU could convert the SM messagesfrom Message Handlers and/or the Packet Switching Unit (PSU) of the 5ESSSwitch Module to a format suitable for routing across the ATM-CM. Astandard ATM Adaptation Layer could be used for this purpose. The ATMUalso provides transport for the ATM-CM Processor Intervention (CPI)function that can be used to force a restart of an SM if a situationarises that requires a reset.

Signaling System 7 (SS7) messages are handled by virtual paths of thetransit network 10, thus providing switch to switch messaging withoutthe need for an intervening STP for routing messages. (An STP, at leastinitially, is still desirable for use in accessing data bases, byproviding data base access to the signal transmission network 10.) TheATMU implements this function by assembling packets into ATM cells,associating a virtual path with each switch SS7 Point Code and thentransferring the cells to the toll ATM network over the designatedvirtual path.

The preferred embodiment described herein takes the view that the ATMUis closely associated with the ATM-CM, since that appears to be the mostuseful from the toll network point of view. An alternative wherein theATMU is closely associated with the SM is also possible. If the ATMU isclosely associated with the ATM-CM, then the ATMU receives its controlfrom the ATM-CM's controller and the AM. In the event that the ATMU isclosely associated with the ATM-CM, control signals are sent over acontrol link 541 via the ATM-CM from the AM to the ATMU centralcontroller (ATMU CC).

As shown in FIG. 23, the ATM-CM is under maintenance control and circuitcontrol of the AM. The AM acts as the ultimate maintenance master forthe the ATM-CM. In this embodiment, the ATM-CM serves only as an ATMcrossconnect, and provides no processing for either video broadcast,frame relay, or Switched Megabit Data Switch (SMDS). Alternatively, theATM-CM can be directly controlled to accept ATM inputs and to switchsuch inputs to a desired destination. This arrangement is useful, forexample, for switching such broadband signals as high definition TV(HDTV) signals, which have a bandwidth that makes direct connection tothe ATM-CM more economical. The AM or another special processor can beused for controlling the establishment of ATM paths for these services.

The assumption is made herein that computers or other terminals thatsend or receive data have reserved adequate bandwidth for sending andreceiving such data before making a request for a connection through anATM-CM.

FIG. 24 is a block diagram of an asynchronous transfer mode interfaceunit (ATMU) 540. The ATMU is under the overall control of an ATMUCentral Controller (ATMU CC), which receives control signals overcontrol links 541 from the ATM-CM. The inputs are from a time slotinterchange unit 517 of one or more switching modules 510. The outputsare to the common broadband platform (ATM-CM) 550. The ATMU isconsidered an ingress and egress node of the overall network, and a PVCinterconnects two ATMUs. This allows traffic from several SMs to becollected for transmission over one PVC from one ATMU to one ATMU. Theoutputs of time slot interchange units 517, which are groups of NCTlinks, enter space switch 610 which has 42 outputs leading to cell widebuffer 620 (CWB). Cell wide buffer stores 9 bit segments, each segmentcomprising an 8-bit PCM sample and one E-bit for the associated channel.Each NCT link carries 512 16-bit time slots every 125 μs. The 16 bitsinclude 8 PCM or user data bits, 7 internal control bits, including asupervisory signaling bit (the E-bit) and one parity bit. All but the 8PCM bits and the E-bit are discarded before an ATM cell is formed. CWB620 includes 42 separate segment-organized memories whose outputs canthen be used in parallel to form the 48-byte payload of an ATM cell. Thespace switch is used to switch the outputs of the NCT links to theappropriate one of the 42 virtual path memories 621, . . . , 625. The48-byte parallel output and a 5-byte output, representing a header, fromthe cell list processor 630 enter one of 8 shift registers 651, . . . ,652. The particular shift register is selected by one of the selectunits 653, . . . , 654 under the control of cell list processor 630. Theoutput of each of these shift registers goes via one of the CBR/VBRselectors 663, . . . , 664 to a Line Processing Unit 661, . . . , 662(LPU), each LPU generates a SONET/ATM data stream. These 8 data streamsare then switched in ATM-CM unit 550. The term ATM-CM as used hereinrefers to an ATM crossconnect switch, having, in this case, ATM/SONETinputs and outputs. The treatment of VBR cells is discussed furtherbelow. Details of the treatment of packets for VBR cells are provided inthe descriptions of FIGS. 28 and 29.

The number of NCT links which can be terminated on one space switch islimited by the speed of the CWB memories and the Cell List Processor(CLP). If several SMs are terminated on one ATMU it is desirable tomaximize the quantity; in the preferred embodiment 24 NCT links areused, but a larger number, such as 60, appears feasible with presenttechnology.

FIG. 25 illustrates the space switch 610. 42 selectors 701, . . . , 702,each controlled by a control memory 703, . . . , 704 are used to switchthe outputs of the incoming NCT links to the appropriate one of the 48virtual path memories that form a cell wide buffer. Each 9-bit switchedsegment (1 PCM sample plus the E-bit) in each of the NCT links may go toany one of the 42 positions in the cell wide buffer. In addition,selector 710, under the control of control memory 711, is used to steer(packetized) variable bit rate data including signaling and othermessages to Message Layer Device 670 (MLD) (FIG. 13). The MLD convertsmessages into ATM cells which are transmitted by the cell list processor630 a CBR/VBR selector 663, . . . , 664 into one of the LPUs 661, . . ., 662 into the ATM-CM after the CBR cells have been transmitted for agiven 125 μs.

FIG. 26 illustrates the cell wide buffer 620. It comprises 42 9-bit widememories, 621, 622, . . . , 623 each comprising a 9-bit by N buffer 801and a control memory 802, where N represents the depth (i.e., number ofcells that can be stored) of the buffer. In accordance with well-knownprinciples of the prior art, in order to preserve frame integrity, thetransmit cell wide buffer is a duplex buffer, one part being loadedwhile the other is unloaded; the receive cell wide buffer is triplex tosolve jitter and frame integrity problems. The control memory steersbytes from the incoming NCT bus to the appropriate position in thebuffer. In addition, the system is arranged to transmit a pseudo randomcode in idle DS0 time slots to test continuity of DS0 channels over ATMfacilities; in one embodiment, the control memory of a CWB is arrangedto insert and to detect the presence of the code under the control ofthe ATMU CC. Alternatively, tones from tone sources in the SM can betransmitted over DS0 channels and detected at the far end.

FIG. 27 illustrates the Cell List Processor (CLP) 630, Facility ShiftRegister (FSR) 651, and Line Processor Unit (LPU) 661. The CLP 630simultaneously reads the CWB 620 while controlling the Selector 653.This causes one 48 byte CBR cell to be written into the SR 651. Inaddition, the CLP outputs the 5 byte header into the SR at the sametime. Thus a full 53 byte cell is loaded into the SR. The cell is nowshifted into the LPU 661 via CBR/VBR selector 663. The LPU 661 transmitsthe cell onto the SONET facility to the ATM-CM.

FIG. 28 shows the Message Layer Device 620. The MLD 620 receivesmessages on NCT time slots from the Space Switch 610 into InterworkingUnits 1020, 1022, . . . , 1024. These messages could be inter-SMmessages, SS7 messages, or user generated messages such as CCITT X.25messages. The Inter-Working Units (IWUs) determine the correctpre-provisional ATM Virtual Circuits identifier, and segments themessage into ATM cells per CCITT specifications using the determined VCidentifier and other header fields as described by CCITT ATM AdaptationLayer specifications. These cells are subsequently shifted out of theInterworking Unit into the CBR/VBR selector and LPU 661 shown in FIG.16, under the control of the CLP. In the preferred embodiment, theoutputs are joined and sent to one or more of the CBR/VBR selectors.

FIG. 29 shows an Interworking Unit 1020. NCT time slots are connectedvia the selector 1110 to the data link controller 1120. A Data LinkController 1120 processes bit level protocol that includes flags, bitinsertion, and CRC. A second controller 1140 processes SS7 or linkaccess protocol within the messages. A processor 1130 determines theVirtual Path to be used for the message, and commands the ATM AdaptationLayer (AAL) Processor 1160 to segment the message into ATM cells. TheATM cells are placed, under the control of ATM Adaptation LayerProcessor (AALP) 1160, into the Cell Buffer i 170 (not to be confusedwith Cell Wide Buffer 620 (FIG. 24)) where they are later transmittedunder control of the CLP 630 (FIG. 16) into the CBR/VBR selector 663(FIG. 27). High priority cells are inserted into Cell Buffer 1170 beforelow priority cells. Cells from the cell buffer 1170 (FIG. 29) constitutethe VBR cells shown in FIG. 8 (the 125 μs frame). The cell buffer mayhave to be several cells deep to take care of VBR cell bunching from theATM-CM.

The AM serves to support the entire 5ESS switch and ATM-CM (includingATMUs) OAMP needs. These include download and control of the ATM-CM,craft graphical display, and communication via ATM with SMs. FIG. 30shows the AM/ATM-CM system architecture as comprising the followingcomponents:

ATM Management Module (AMM) including directly connected terminal.

This is an adjunct hull tolerant processor that connects to the existing5ESS switch AM, and serves to provided added processing throughput fornew ATM-CM and ATMU capabilities.

Ethernet® Bus to interconnect AM/AMM with Graphical User Interface(GUI), ATM Packet Handler (APH), and ATM-CM.

Small Computer System Interface (SCSI, an industry standard) peripheralsfor disk, tape, and Compact Disk Read Only Memory (CD ROM) on-linedocumentation: These augment the existing AM non-volatile peripherals.

GUI workstation terminals that supports existing 5ESS switch equipment,ATM-CM, and ATMUs.

ATM Packet Handler provides the AM/AMM with the ability to communicatevia ATM over SONET to the SMs. The SMs terminate the APH's ATM in theirATMU MLDs. To communicate with SMs, the AM/AMM sends messages viaEthernet to the APH which performs the message to cell conversion andtransmittal to ATM-CM over SONET.

The GUI and non-volatile memory are commercial components whose controlresides in AMM software. The design of the AMM and. APH components areexpanded in Section 5 of the Detailed Description.

FIG. 31 illustrates the circuit (channel) hunt which is performed by aswitching module processor 511, or other processor having data about thestatus of PVPs from the connected ATMU to the destination of the call.This processor receives a path request (action block 1200) anddetermines (test 1202) if there are any available circuits (channels) ondirect active virtual CBR paths to the destination of the path request.If so, then an available circuit is selected (action block 1204) and amessage is sent to the node (typically, a processor for another ATMU) atthe other end to notify that node that a circuit has been established ona particular slot of a particular active CBR PVP.

If no available circuits on direct active CBR PVPs are found in test1202, then test 1208 is used to determine if there are any availablecircuits on alternate active CBR PVPs. If so, then an available pathfrom one of these alternate active virtual paths is selected (actionblock 1210) and the node at the other end is notified (action block1206). (An alternate active virtual path is an active virtual path usingan alternate route which is a route that uses at least two links insteadof the single link direct route.) If no circuits are available on activevirtual paths for this circuit request, then a request is made toallocate an additional virtual path (action block 1212). This request issent to the administration module 530 (FIG. 12) which activates anadditional virtual path as described in FIG. 32. Eventually, theadministrative module responds to SMP 511 with a success or failureindication and, in the case of a success indication, the identity of theallocated virtual circuit. Test 1214 is used to determine that successor failure. If the allocation request has been successfully responded to(positive output of test 1214), then test 1202 is reentered in order toperform the process of selecting an available circuit. If the allocationprocess was unsuccessful (negative output of test 1214) then an allpaths busy treatment is given to the call for which the path request wasoriginally received in block 1200.

It is assumed in this discussion that the controlling processor, such asthe switching module processor maintains a list of active CBR virtualpaths for carrying traffic outgoing from its associated switching moduleand maintains an activity state for each channel of such a virtual path.It is, of course, also possible to maintain this information elsewheresuch as in the administrative module, but the suggested arrangementminimizes the time required for establishing most calls. Further, it ispossible to use virtual CBR circuits with two-way channels but such anarrangement, while it allows for more efficient use of the virtualchannels, requires negotiation between the two endpoints in order toprevent "glare" (i.e., a situation wherein the same channel is seizedconcurrently by the two end nodes connected to the channel).

FIG. 32 describes the process of activating a virtual CBR path, or for asplit path, a group of channels. Many more virtual CBR paths areprovisioned (i.e., stored in memories of the network) than can be activeat any one time. Basically, virtual CBR paths are provisioned to handlethe peak traffic between any pair of nodes. Virtual CBR paths areconsidered activated when they are available for carrying traffic. Theprocess of activation is designed to ensure that the physical ATMfacilities carrying the virtual CBR paths are not overloaded and that nodefective physical ATM facilities are used to carry traffic. In case ofa failure, for example, all virtual CBR paths that use the failedfacility must be deactivated.

In this specific embodiment, the process of activating additionalvirtual CBR paths is analogous to the process of seizing individualtrunks in accordance with the teachings of the real time routingarrangement as described in G. R. Ash et al.: U.S. Pat. No. 5,101,451,by routing additional traffic over less heavily loaded transmissionfacilities, in this case, SONET/ATM facilities. One specialcharacteristic of the arrangement described herein which is notanalogous to a situation encountered in routing traffic over individualtrunks is the use of split groups, illustrated in FIG. 10, i.e., activevirtual CBR paths a portion of whose channels are used for outgoingtraffic in one direction and another portion of whose channels are usedfor outgoing traffic in the other direction. Such split groups areespecially efficient for use in carrying traffic between two nodes forwhich the level of traffic is relatively low. Another difference is thatmore links may be required for alternate routed traffic so that the loadof several links may have to be considered in selecting an alternateroute PVP for activation. A third difference is that PVPs using anintermediate CCR, as shown in the middle CCR of FIG. 4 and of FIG. 11,can be used as alternate routes, equivalent to "2-link routes" of thecited patent; such alternate routes are relatively inexpensive sincethey do not require traversal of an intermediate access switch.

An administrative module receives an allocate request from a switchingmodule processor 511 (action block 1300). The administrative modulefirst determines (test 1302) if there any virtual CBR paths carryingsplit traffic (i.e., outgoing from the two end nodes). If so, theadministrative module determines how many channels are currently busy ineach direction and checks if there is adequate margin in the split groupto allow for an additional few channels to be allocated in the directionassociated with the allocate request. The channels of split groups arearranged so that the first n channels are hunted in one direction andthe remainder in the other direction and that the hunt for a channel isperformed in such a way as to keep the middle channels availablewhenever possible. If middle channels are available and if the number ofthese available channels is sufficiently large to allow for a movementof the division point (positive result of test 1304) then the splitpoint is moved (block 1306) and the two end nodes are so informed(action block 1308). The requesting node is informed of a success inresponse to the allocate request and when the requesting SMP retriestest 1202 and 1208, one of these will now pass. In this embodiment splitgroups are checked first; simulation studies may show that thealternative of checking for available additional PVPs first, is moreoptimum.

If no split groups are available (negative result of test 1302) or ifthere is inadequate margin in the split group(s) (negative result oftest 1304) then test 1320 determines if there is an available idleprovisioned direct virtual CBR path. If so, it is necessary to checkwhether activation of an additional CBR PVP will cause congestion on anylink carrying that PVP. If such congestion is found, that idleprovisioned direct virtual CBR path is rejected and not activated, andtest 1324 is tried; if no congestion is caused, that path is activated(action block 1322) and the two end nodes of that path are informed ofthis activation (action block 1308). Otherwise, test 1324 is used todetermine if there are any alternate available virtual CBR paths. Inmaking the choice among available alternate CBR paths, the principles ofreal time network routing are used by preferably selecting availablealternate virtual CBR paths that use relatively lightly or less heavilyloaded ATM facilities. In making the determination of which facilitiesare less heavily loaded, since two link circuits are preferred over3-link circuits, the loading of the potential 2-1ink circuits can beascertained by requesting the administrative module connected to theATM-CM that is connected to the destination switching module to reporton the loading of its ATM circuits. Since each activation of a virtualCBR path represents a larger utilization of resources (48 channelsversus 1 channel) than is the case for the selection of a single trunkfor real time network routing, a loading threshold limit beyond whichvirtual CBR paths should not be activated should be applied for anyfacility that is used by a candidate CBR PVP. The limit is also affectedby the amount of VBR traffic supported by the ATM facilities. Clearly,this limit is a parameter which should be under control of the networkadministrators, which may be different for different ATM facilities, andwhich should be adjusted as field experience is obtained.

Note that in the preferred embodiment, all provisioned PVPs have apredetermined path traversing two end nodes and a variable number ofintermediate nodes. Alternatively, switched virtual paths could beprovided having a variable path selected at activate time.

If no available alternate CBR virtual paths are available forassignment, then the allocate system returns a failure indication to therequesting SMP (action block 1326). If an available alternate path hasbeen chosen (action block 1328) (the available circuit is selected amongalternate circuits with relatively low load) then the alternate path isactivated (action block 1330) and the end node and intermediate ATM-CMswitches are notified of the activation of the CBR virtual path.

The active CBR PVP consolidation process will now be discussed. In thecourse of normal toll call activity, it usually happens that differentcomposite cells between two end nodes are not completely filled.Furthermore, it is a frequent occurrence that the total number of DS0sin N active composite cells is less than or equal to the total number ofDS0s supported by N-1 composite cells (48 N-N). In that case it isdesirable to consolidate the PVPs carrying these cells to form morefully utilized cells that carry the DS0s. The PVP for composite cellswhich no longer have active DS0s can then be made inactive, thus freeingbandwidth on one or more ATM facilities. That freed bandwidth can thenbe used for composite cells between other sources and destinations.Thus, more efficient utilization of the network is achieved. Theconsolidation process is described in FIG. 33.

In the discussion that follows the composite cells being consolidatedwill be discussed as being between ATMUs, and the processing ofconsolidation is performed by the ATM-CM controller. Alternatively, itis also possible for the AM or SM processor (SMP) to control theconsolidation of composite cells instead of the ATM-CM. However, theprocessing would be the same in either case, only the identity of theprocessor would be different.

To perform consolidation, processors continually check active compositecells for a partial fill condition which is defined to be N compositecells to the same destination such that the total number of active DS0swithin N cells is less than the total number of DS0 in N-1 compositecell (i.e., (N-1)*48). The amount by which the sum of the activechannels in the N cells is less than (N-1)*48 is a parameter whichshould be set as a result of simulation and experience. If the parameteris too low, say zero, thrashing may occur; if it is too high, say 10,inefficient use of facilities will occur. For this discussion a cell andits corresponding PVP are used more or less interchangeably. Each activecell is under the control of one connected end node, the end node thatoriginates outgoing traffic, or, in the case of split traffic cells, anode selected arbitrarily. When the controlling processor determinesthat a partial fill condition exists, it communicates a request to thedestination node to consolidate the N composite cells. The requestcontains:

The identity of the N composite cells (e.g., via the Virtual PathIdentifier). N-1 cells are the consolidated cells, and one cell is thecell to be eliminated (since it will carry no DS0s followingconsolidation.)

A list of DS0 cell re-ordering from the cell to be eliminated to theconsolidated cells.

In this embodiment, DS0s of the consolidated composite cells remain inthe same cell locations, and DS0s from the eliminated cell are moved toidle positions in the consolidated cells.

The other node responds with an acknowledgement to consolidate. Duringthe consolidation period, new calls that require transport between thesetwo nodes (typically switching modules) will only use DS0s from theconsolidated cell that are not to be filled from the cell to beeliminated. It is possible that calls could arrive and therefore triggerthe activation of a PVP for one composite cell while another PVP isbeing eliminated. This unlikely occurrence is in accordance with theinvention since eventually excessive PVPs are eliminated.

In order to consolidate, newly arriving DS0 signals for the cell to beeliminated are sent into both the consolidated cell and the cell to beeliminated. Within the ATMU, a DS0 on an NCT link from the SM is readfrom the memory at two discrete times to be placed into two differentcells and DS0 locations.

The processor performing consolidation commands the ATMU CC to performthis function. After performing this action, the processor sends amessage to the other node indicating that the aforementioned copy actionhas transpired. The other node must send a similar message to theoriginal node. Upon reception of this message, the destination processorcommands the ATMU CC to deactivate the PVC of the eliminated cell. TheATMU CC causes the ATMU to read DS0s from the buffer locationsassociated with the consolidated cells instead of locations associatedwith the eliminated cell. The ATMU CC then removes this cell from theCLP's active composite cell list. The eliminated composite cells are nolonger sent or processed into DS0s on the NCT links. Since the DS0s areswitched via the ATMU to the NCT links, there is no change of time slotson the NCT links to the SM TSI (Time Slot Interchange). The other endthen also deactivates the PVP of the eliminated cell.

A problem arises if a channel from one cell is transferred into the samechannel in another cell; during the period of transition, this cellwould be written into the same channel position of two buffer locations.This problem can be overcome in one of three ways:

The speed of the CWB can be doubled to allow two writes to occur in oneinterval

the consolidation can be arranged through software to avoid such atransition

the transition can be made in two stages, first by freeing anotherchannel of the transferred-to cell by copying a busy channel to the idleposition and then releasing the previously busy channel, then bytransferring the channel from the cell being released to the newlyreleased channel of the target cell.

If two or more SMs are connected to one ATMU, either the SMP of one ofthe SMs controls the PVP being deactivated or, preferably, the AM of theconnected ATM-CM controls the PVP. In either case, only a singleprocessor controls the consolidation process at the controlling node.

A flow chart for implementing the consolidation process is shown in FIG.33. At some point, the consolidation process is started by the processorwhich controls allocation of permanent virtual paths (action block1400). A test is made to determine the existence of partial fillpermanent virtual paths (test 1402). This test is performed by checkingto see if there are N PVPs with a common source and destination nodewhich in combination carry less traffic than can be carded by N-1 PVPs.Since there is normally an order to the search for PVPs to a particulardestination, the partial fill PVPs are most likely to be found at theend of such a list. As each source-destination set is checked if thepartial fill condition does not exist (negative result of test 1402), acheck is made to see if the process is complete (test 1404) and if so,this is the end of the consolidation process at this time (end block1406). If not, another set of PVPs is checked for the partial fillcondition. (In general, it is expected that no more than one PVP wouldcarry traffic initiated from both ends. A PVP carrying such traffic canbe eliminated in two steps by first eliminating the outgoing channelsfrom one direction and after these channels have been eliminated in theprocess described hereinafter, convening the cell to be eliminated, notto a cell of an inactive PVP, but to a cell of a PVP carrying thetraffic that is now fully controlled by the other end node.)

If a partial fill PVP set is found (positive result of test 1402), andit is found that the total traffic carried over the route served bythese PVPs can be adequately served by one less PVP (to eliminateexcessive shuttling of PVP activation-consolidation), then thecontrolling node (carrying outgoing traffic for that PVP) consolidateschannels from a cell to be eliminated into the target cell (action block1410). A message is then sent to the far end node (the node withincoming traffic for that PVP) informing that node of the new channelsto be occupied by the channels from the PVP which is to be deactivated(or, in the case of a two-way traffic PVP, to be converted into aone-way traffic PVP). The outgoing node also arranges to have trafficfrom the channels of the cell to be eliminated transmitted to both thecell of the PVP to be eliminated and the consolidated target cell(action block 1414). The outgoing side node then waits to receive apositive acknowledgment message from the incoming traffic node (action1416). In response to that message, the PVP corresponding to theeliminated cells is deactivated (action block 1418) (or, in the case ofa two-way PVP, that PVP is then converted into an incoming traffic onlyPVP). Following execution of block 1418, other PVPs are checked for thepartial fill condition (test 1402).

Detailed Description of ATMU

This section discusses the high level design of the AsynchronousTransfer Mode Unit (ATMU) that is used to perform the following twogeneral functions and their reverse:

Conversion of DS0 Transport to ATM Composite Cells

Conversion of Variable Length Data Messages to ATM Cells

This overview focuses on DS0 flow from the 5ESS switch to the ATMnetwork (CBR traffic).

Space Switch

A block diagram of the ATMU is shown in FIG. 24. Network Control andTiming (NCT) links from the SM Time Slot Interchange TSI 517 are shownterminated on a space switch within the ATMU. The purpose of the spaceswitch is to fan out the NCT links to 42 internal links. These linksterminate on an array of cell memory devices, called the Cell WideBuffer (CWB) in FIG. 24, that store the 42 DS0 PCM samples and E-bits ofa composite cell. Thus, the space switch serves to route time slots(DS0s) to the correct composite byte location on the input of the CWB.

Cell Wide Buffer

The function of the Cell Wide Buffer (CWB) is to assemble the segmentsof the virtual path cells in a format which allows them to besimultaneously read out in a single memory read cycle. The CWB is abuffer whose width is the size of one ATM cell (not counting headerbytes). The width is 42 segments (a segment, as used herein, is an 8-bitbyte plus an E-bit, and the depth equals the number of unique activevirtual paths that needs to exist at a given moment. As shown in FIG. 24each of the PCM byte positions in the cell is written into separatememories each of which has independent write control circuitry. Eachmemory address in these memories corresponds to a specific virtual path.Each of these byte memories has a control memory that maps individualtime slots (only the PCM part plus the E-bit) to one virtual pathsegment location in the CWB. Since the control memory for each segmentmemory is independent, each active time slot on the 42 links is able togo to any virtual path memory location in the buffer. Thus, bytes on the42 links that appear at the same time at the input to the CWB can bestored in different virtual path memory locations in their respectivebuffer memories and thus assembled into different virtual path cells inthe CWB. The control memory is able to block a write to the byte buffermemory if a particular DS0 on an NCT link is not active i.e., notpresently in use for a any call.

Cell List Processor

During every 125 μs interval all active DS0s are written into theassigned byte locations of their selected virtual path cell. Compositecells are read from the cell wide buffer every 125 μs under control ofthe Cell List Processor (CLP) 630, shown in FIG. 24. The CLP has a listof active composite cells that are stored in a linked list. The linkedlist stores the ATM virtual path address header bits, and the CWBaddress that holds the virtual path's composite cell. The CLP list onlystores the virtual paths for which active DS0s exist. The CLP traversesthe list once every 125 μs, thereby causing each composite cell to betransmitted. After the CBR cells are transmitted, the CLP reads VBRcells (if any are queued) from the Message Layer Device (MLD) 670 to useup spare time in the 125 μs interval, as illustrated in FIG. 8.

Conversion to Synchronous Time Multiplex (STM)

Since the cell wide buffer is the width (data part) of an ATM cell, anentire ATM cell can be read from the cell wide buffer in one access.Parallel to serial conversion is implemented by means of parallel-inserial-out shift registers 651, . . . , 652 which are selected by meansof the Shift Register (SR) Selector 653, . . . , 654 under control ofthe CLP 630. The read data along with ATM header data from the CLP isloaded into a shift register that connects to a SONET facility interface661, . . . , 662. In the case of a 46 byte cell, the two byte index isalso provided by the CLP 630. The SONET facility interface connectsdirectly to the ATM-CM.

ATMU Control

The control of the ATMU is provided by the ATMU central controller (ATMUCC) 601. Control messages are received in the ATMU CC from dedicatedtime slots intercepted in the space switch and transmitted to the ATMUCC over a dedicated control bus (not shown). The control messages aretransmitted from a switching module processor (SMP) 511 of the 5ESSswitch module (SM) 510 via a Message Handler 513 in the SM. The controlsignals from the ATMU CC are distributed in the ATMU over a group ofcontrol buses. The Space Switch 610 is provided with path set up andmessaging control memory information, active link selection and activeside selection. Link selection is used to determine which NCT links fromthe SM are active. Alternatively, the E-bits, discussed hereinafter, canbe used to identify the active links since E-bits are only active foractive paths. Link selection is used to determine which NCT links fromthe SM are active. The links can be either active or standby. The sideselection determines which side of the ATMU is active or standby. TheCell Wide Buffer is provided with the mapping of the time slots tovirtual paths and allocation of byte positions within the ATM cell. TheCell List Processor is provided with active virtual path information andprovides the Shift Register Selector 653, . . . , 654 with shiftregister address information. In this ATMU design, the ATMU can supportmore than one SONET facility, and so each ATM cell can go to one of thepossible 8 Shift Registers. The CLP is provided with the identity of theproper shift register for each virtual path. The Inter-Working Units(IWU) 1020, 1022, . . . . , 1024 (FIG. 17) of the MLD are provided withthe provisioned virtual path destination addresses also. The ATMU CCprovides the maintenance control of the ATMU. Maintenance messages forthe ATMU CC itself such as initialization of the ATMU CC are receivedover the dedicated control bus before entering the space switch.

The ATMU is controlled by the SM in the preferred embodiment.Alternatively, the ATMU can be controlled by the ATM-CM. In that casethe ATMU Controller receives control from the Common BroadbandPlatform's Controller (CBP). The ATMU provides SM to SM, and SM to AMmessage based communication services for the 5ESS switch viainterprocessor packets.

Message Interworking

The ATMU provides the following message based communication services forthe 5ESS switch:

SM to SM, and SM to AM via interprocessor packets

SS7 Message Transfer Part (MTP) packet transport

Both of these functions are handled in a similar manner. In FIG. 24, theMessage Layer Device (MLD) receives messages from the SM via the spaceswitch. Variable length 5ESS switch and SS7 messages are carried in timeslot channels that originate from protocol handlers (PH) in the PacketSwitching Unit (PSU) or from the SM Message Handler. The MLD containsthree types of Inter-Working Units (IWU), the 5ESS switch messaging IWU,the SS7 IWU whose function is to reconstitute messages from the SMP, andthe inter-PSU IWU for handling user to user data messages. The functionof these units is to:

Accept messages carried from the space switch

Associate a virtual path that connects to the destination identified inthe message's header

Perform ATM segmentation and reassembly

Shift data, when instructed by the CLP.

The handling of SS7 is slightly different than 5ESS switchinterprocessor messages. For 5ESS switch interprocessor messages, theframe is relayed in a virtual path to the destination AM or SM. Themessages are processed to read the destination address. The destinationaddress determines which Virtual Circuit the ATMU uses for segmentationto cells.

In the SS7 message case, the signaling data link is terminated in theSS7 IWU, and the MTP message is relayed on a virtual path to thedestination toll office. The handling of cell based messages coming fromthe ATM network and general control and fault tolerancy are discussed indetail hereinafter.

For inter-PSU messages, the messages are processed to read thedestination PSU address; this destination address determines whichvirtual circuit the ATMU uses for segmentation into cells.

SM/ATMU Remoting via SDH/SONET

In this embodiment, the ATM-CM and ATMU are connected via intraofficeSDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical NETwork)facilities that carry ATM. The SM with ATMU is not a Network Element asviewed by the SDH or SONET network, and does not terminate the digitalcommunications channel (DCC) section overhead. However, the intra-officefacility is optical, so that an SM with ATMU can be remoted from theATM-CM. If the SM with ATMU connects directly to the ATM-CM, then onlyfibers (and, where appropriate, repeaters) are needed for such remoting.

In the event that an SM with ATMU is remoted via the general SDH/SONETnetwork, then SDH/SONET multiplexors or crossconnects that supportSynchronous Time Multiplex-1 (STM-1) and multiplexed STM-1 are used inorder to properly terminate section DCC at the SM end. In this generalcase the SONET/SDH facility at the SM end must be terminated by aSONET/SDH multiplexor in order to provide an intra-office SONET/SDHfacility directly to the ATMU. This will make it unnecessary to developseparate SDH/SONET DCC Operations Administration MaintenanceProvisioning (OAMP) functions in the SM and avoid the possible confusionof having both the CBR and the SM performing SONET/SDH OAMP processingin the same office.

The above discussion applies only to the SM to ATM-CM links (assumingthat the ATMU is part of an SM). The ATM-CM terminates section DCC forinteroffice trunks, and is viewed by the SDH/SONET network as a generalNetwork Element. Thus, due to the capabilities of the ATM-CM, the 5ESSswitch (i.e., an AM, a group of SMs, their associated ATMUs, and anATM-CM) is an SDH/SONET Network Element even though the SM by itself isnot.

Alternatively, it is possible to associate a group of ATMUs directlywith an ATM-CM instead of directly with the SMs. In this case, the NCTlinks connecting the SMs with ATMUs are longer as in the well-known caseof optically remoted SMs for the 5ESS switch. In that case, a SONETfacility can be used to convey the ATM signals to the ATM-CM inessentially the same way as distant ATM signals are conveyed thereto, orif the ATM-CM can be so arranged, the ATM signals can be directlytransmitted from the ATMU to the ATM-CM.

ATMU General Functions

This section summarizes the functional relationship of the ATMU to theATM-CM and SM. The ATMU is treated by the connected SM(s) as anintelligent peripheral unit that receives control messages from theSMP(s) in the same way that other intelligent units do. The purpose ofthe ATMU is to provide:

1. SM time slot to ATM composite cell conversion assuming fixed,provisioned virtual paths. Up to about 10,000 time slots (20 NCT links)are supported using present technology. (An NCT link transmits 512multiplexed DS0 bit streams, and uses optic fiber transmission.) TheATMU can route any incoming time slot to any byte position of anyconnected active CBR virtual path. The active CBR virtual paths are aproper subset of a large number of pre-provisioned virtual paths, mostof which are not active at any one time.

2. Inter-SM and SM to AM and/or SS7 variable length message to ATM cellvirtual path conversion, using fixed provisioned VBR paths. Because ofthe high priority of SS7 messages, it is desirable to pre-allocate VBRbandwidth for the SS7 signal paths. The ATMU transports inter-SM LinkAccess Procedures (type) B (LAPB) frames without any termination of LAPBprotocol. In the case of SS7, the ATMU terminates the SS7 Level 2(High-level Data Link Controller (HDLC Part)), and transports MTP/SCCP(Message Transfer Part/Signaling Control and Connection Part) messages.The MTP protocol is not terminated at the ATMU.

3. An SDH/SONET facility access to the ATM-CM is provided for thevirtual path cells. The rates are STS-3 (Synchronous Time Signal) toSTS-12 and STM-1 to STM-4 for SONET and SDH respectively, and asufficient number of these facilities is provided to meet outgoingcomposite cell needs for an SM supporting up to 10,000 trunks. For moreor fewer trunks, more or fewer facilities can be equipped.

4. Handling of NCT A-G bits (bits that are sent with each PCM samplefrom the SM to the ATM-CM) that is as transparent as possible to the SM,including E bit (supervision of a time slot) functions. Time slot parityon the NCT is also terminated/generated.

5. Support Central Processor Intervention (CPI)

This is a function that sends special bits on the NCT link that willreset the SM's processor, in case that processor loses sanity.

6. Support inputs from a plurality of SMs.

7. Support inter-PSU user to user message traffic.

ATMU Components

This section discusses the design of the individual blocks of the ATMUand the duplication plan to achieve high reliability:

Space switch 610

Cell Wide Buffer 620

Cell List Processor 630

SS7 message Inter-Working Unit 1022

Inter-SM/AM message Inter-Working Unit 1020

ATMU Central Controller (ATMU CC) 601

The first three blocks of the ATMU listed involve DS0 to composite cellconversion. The next two blocks involve variable length message to ATMcell conversion. For purpose of architectural discussions, the SS7 andintra-switch IWU are considered to be part of the Message Layer Device.

Discussed after the ATMU CC section are alternatives for implementingthe CBR Processor Intervention capability of the ATMU (with CM-ATM).

Space Switch

The space switch interconnects NCT link DS0s from the SM ModuleController Time Slot Interchange (MCTSI) with NCT bus DS0s to the CellWide Buffer (CWB) and the ATM Message Layer Device (MLD). The number ofNCT buses to the CWB is 42. There also is one NCT bus to the MLD. Byconvention, NCT buses on the "SM MCTSI side" are called links, buses onthe "CWB side" are called NCT buses. Anywhere from 2 to 24 NCT links(i.e., up to 10,000 trunks) (or more in a next generation SM) can besupported by the space switch. The internal design of the space switchis shown in FIG. 25. The fabric is the core of the design, and consistsof multiplexors that accept up to 24 NCT links. These multiplexors havea control memory that selects one of the up to 24 NCT links for each ofthe 512 locations on the NCT link. The selected NCT link's time slot isthen the output of the multiplexor for the given time slot period. Thenumber of multiplexors is equal the number of segments in the compositecell (i.e., 42 multiplexors) plus one more multiplexor to create the NCTbus to the MLD. Thus there are a total of 43 multiplexors depending onthe composite cell chosen for the overall system. In this manner, any ofthe 512 time slots on any of the NCT links can be connected to any ofthe byte locations in the cell wide buffer or the MLD.

The NCT links from the MCTSI terminate on the NCT Link Interfaces (NLI).The NLIs present a synchronized set of backplane buses to the spaceswitch multiplexors so that all multiplexors are switched synchronously.The multiplexors then present a set of synchronized buses to the CWB andthe MLD.

In the reverse direction (i.e., from CWB towards SM MCTSI) the spaceswitch operates operates in exactly the same manner. Bytes from the CWBterminate at fabric multiplexors driven by control memories. The outputof these multiplexors connect to NLIs that then cross couple to SM MCTSIhardware.

The core fabric of the space switch interfaces to the NCT links via theNLI cards. The NCT links are duplex, meaning they originate from eachside of the SM MCTSI. In order to cross couple to the SM MCTSI, the NLIsthemselves are duplicated. The NLIs are part of the core fabric failuregroup, so that each side of a NLI connects with one multiplexor fabricside. The NLIs are coupled to both sides of the SM's TSI, but attach toonly one side of the space fabric. Thus, either space fabric side canreach either SM TSI side. The multiplexor fabric cards connect directlyto the CWBs. The NLI, multiplexor fabric, and CWB are all in the samefailure side. As discussed below, the Cell List Processor is also inthis same failure group. From a system perspective, the cross coupledNCT links terminate two common sides of the ATMU, each of which whichhas a space switch plus associated hardware to form the ATM cells.

The NCT buses are coupled via the space switch to the MLD. The MLD iscross coupled with the space switch, and is in its own failure group.

The space switch (with NLI) uses the following error detection methods:

Parity on NCT time slots

Parity on control memories

Parity on control buses from the ATMU CC

Loss of clock and synchronization

Errors from these detectors are send to the ATMU CC, which then executesfault management procedures.

Space Switch Control

The control memories in the multiplexors, as well as other controlregisters used to select active NCT links and ATMU sides, are all undercontrol of the ATMU Central Controller. The space switch hardware has nofirmware or software; all control is provided via buses from the ATMUCC.

There are two operational uses of the space switch, both of whichrequire space switch memories to be written by the ATMU CC, namely callcircuit switching and provisioning of message DS0s from message sourcesin the SM to the MLD. For call circuit switching, when a call is set up,signals for one DS0 must be connected to a cell wide buffer virtual pathbyte location. The SMP (in conjunction with the AM) determines thevirtual path byte location to use for each DS0 signal. The SMP causeseach DS0 signal to be placed into an NCT link time slot using an MCTSITSI slice, and then orders the ATMU CC to write the appropriatemultiplexor memory to route that DS0 signal to the correct CWB bytelocation. Cell wide buffer hardware then moves the DS0 signal to thecorrect virtual path cell in the memory.

The SMP provisions DS0 channels to carry messages from the MessageHandler or PSU, through an MCTSI slice to the space switch. The SMP thenorders the ATMU CC to write an appropriate multiplexor location thatroutes the DS0 channel(s) to the MLD.

In the above scenarios, the duplicated space switches are written withidentical information.

Cell Wide Buffer (620) (FIG. 26)

One of the two output destinations of the space switch is the CWB 620(FIG. 26). Each segment location in the CWB has a control memory calleda Time Slot Assign (TSA) that loads the virtual path segment for eachNCT time slot received (FIG. 26). The TSA can also be programmed to notaccept a given time slot on the NCT bus. Thus, each cell wide bufferlocation can, on a per NCT slot basis, independently load a different(or identically same) virtual path composite cell. (The serial NCT linksignals shown in FIG. 26 are converted into segments prior to beingstored in buffer 801).

The unloading of the CWB towards the ATM-CM is under control of the CellList Processor (CLP) 630. The CLP makes requests synchronously to theCWB by sending an address and a read request. An entire cell payloadportion is read at once in one memory access into a shift register. Atthe same time, the CLP loads the cell header into the shift register.The shift register is shifted out to an SDH/SONET facility LineProcessing Unit (LPU) 661, . . . , 662. CLP read operations of the shiftregisters are interlaced with the TSA controlled writes from the NCTlinks.

In the reverse direction (from ATM-CM to SM MCTSI) the cell header isunloaded from the shift register into the CLP. The CLP uses the headerto determine the CWB location for the call. (Because of the symmetry ofthe two directions of the paths, all segments of a particular input cellto the ATMU may be stored in a common location of a cell-wide buffer.)Then, the CWB is loaded from the shift register into the buffer.Finally, the TSA control memories in the buffer read the bytes onto theNCT buses at the correct time.

The depth of the CWB until now has been discussed as being equal to thetotal number of (active) virtual paths so that one DS0 per virtual path(in each direction can be stored). However, the buffer must be two orthree times as deep (depending on direction) due to the followingreasons:

The fundamental nature of ATM is that jitter of cells can occur due torandom internal queuing within the ATM-CM.

CWB loading and unloading from the shift registers must be synchronizedwith CLP reads and writes to/from the shift registers to avoid cellsbeing sent that have DS0s from different NCT 125 μs frames. Such framemisalignment could disrupt the continuity of N*DS0 services beingtransported by the composite cell.

To solve this problem, in the direction from the SM MCTSI towards theATM-CM, the CWB is double deep (two partitions). This means that thespace switch loads one half of the CWB, while the CLP reads from theother half. After one partition is written the CLP and space switchaccess the other partition. Each half (partition) is identical incomposition (i.e., has the identical number of virtual circuit compositecells supported). In the direction from the ATM-CM towards the SM TSI,the CWB is of triple depth (three partitions) in order to allow 125 μsof "build-out". Build out is used to keep the CWB from "under-running",and amounts to 125 μs of fixed delay. Under-run occurs if cell jitteroccurs to the extent that no new DS0 exists in the CWB when the NCT bustime slot must read that location. Thus, two of the "partitions" of theCWB provide the buffering to account for the jitter, and the thirdpartition accounts for the time to read the DS0s from the CWB withoutinterference from cell writes due to incoming cells. The controlmemories in the TSA of the CWB automatically cycle through the CWBbuffer partitions. The CLP determines into which partition an incomingcell from the ATM-CM a cell should be written. The partition varies fromvirtual circuit to virtual circuit due to the fact that jitter can cause(e.g.) zero, one or two cells to arrive in a given 125 μs interval. Ifcells are lost, then the triple buffer CWB can wrap around (in effect anunderrun). This is detected if the space switch and CLP ever access thesame partition, and causes the CLP to read from the partition that isfurthest in time from the current build-out partition.

DS0 Path Monitoring/Test

Monitoring circuitry is provided to determine that composite cells arebeing received by the ATMU every 125 μs on average. To achieve this, theCLP maintains a counter for each composite cell virtual path that isactive. Every time a cell arrives, the counter is incremented. Thecounter increments on average every 125 μs. The counter may notincrement exactly every 125 μs due to jitter of ATM cells resulting fromrandom queuing in the ATM-CM. This jitter is a fundamental aspect of theATM-CMs. The CLP determines every 10 milliseconds (ms) whether thecounter has incremented approximately 80 times. This will have avariation of plus or minus 3 due to near term jitter, and very lowprobability cell loss and insertion. If the counter is further from thisvalue, the ATMU CC is notified which is able to read the counter todetermine if virtual path loss has occurred. The counter is able toreach 8000 plus or minus 4. The counter is then checked after one secondto determine if it has reached 8000 plus or minus 4. The 8000 count isused to determine if a high cell loss per second has occurred.

A direct test of circuit continuity is designed into the ATMU. ATMU CWBmemory devices at the source (towards the ATM network) insert a code(possibly multi-byte) into a byte location of a cell. This occurs beforea talk path is connected. At the destination end of the connection, thecell wide memory devices read the pattern and detect a match. This bytecode can be changed as part of the test to cause bits in the DS0 totoggle. This toggling can then be recognized within the ATMU CWB andreported to the ATMU CC. The ATMU CC can then cause E bits on (pathcontinuity bits that are internal to a 5ESS switch and not transmittedover inter-switch facilities) on the NCT link that carries the DS0signal to toggle, as well as send messages to the SMP that indicatedthat continuity has been established. (E-bit use is described in E. H.Hafer et al.: U.S. Pat. No. 4,280,217).

The CWB is in the same failure group as the space switch, as discussedabove so that failures in the cell wide buffer result in a side switchof the CWB, space switch and CLP and the MLD. The LPUs are cross coupledto the facility shift registers (FSRs) and CWB. Thus, a CWB can receivecells from either of the duplicated LPUs. Which of the two LPUs is beingused is under control of the ATMU CC.

The primary error detection method in the CWB is parity over:

NCT buses from the space switch

Virtual path byte memories

TSA control memories

Control buses from the ATMU CC

Control

The CWB is controlled by the ATMU CC. There is no firmware or softwarein the CWB hardware.

From an operational point of view, the CWB is used to map a givencomposite cell byte location to a particular virtual path. The SMProutes a DS0 channel through the SM MCTSI, and then causes the ATMU CCto connect a DS0 through the space switch to a given byte location onthe CWB. The control memory in the CWB for that byte is then writtenwith an address associated with the virtual path. This occurs in TSAcontrol memories for both directions. At this point in time, aconnection exists from an SM MCTSI DS0 and a given composite byte (DS0)on a given virtual path.

All other functions are likewise under control of the ATMU CC, includingerror detection control associated with DS0 continuity test andmonitoring, and LPU state control.

Cell List Processor (FIG. 27)

The CLP 630 (FIG. 27) is responsible for moving cells between the CWB620 and the Facility Shift Registers (FSR) 651, . . . , 652 (FIG. 24.)Towards the ATM network, the CLP has a linked list of records that storethe CWB location for each active virtual path. Every 125 μs the CLPtraverses this list and causes the CWB to load all active cell into theFSRs. In the other direction, the CLP has an address look-up functionthat is used to load the CWB with cells incoming from the FSR. As statedin the description of the CWB, the CLP keeps track of the CWB partitionto be accessed for signal transmission in both directions.

The FSRs connect to the facility Line Processing Unit (LPU) (661, . . ., 662) which is the actual hardware device (circuit card) thattransforms bits for transmission to the physical media, and thatperforms facility related maintenance functions. Thus the LPU cardsupports optics, synchronization, and SDH/SONET overhead processingcircuitry such as Byte Interleaved Parity, etc.

FIG. 27 shows the internal design of the CLP. The sequencer 920 readsservice request bits from the FSR. The request bits can be inhibited ifan FSR does not connect to an equipped or otherwise In-Service LPU. Thesequencer responds to a service request bit for a given FSR by checkinga list of active virtual paths that are provisioned to the facilityassociated with the FSR. The list, stored in the virtual path listmemory 930, accessed by the sequencer, is started at the beginning ofevery 125 μs interval, and must be completed before the start of thenext 125 μs. The last requirement is a reflection of the fact that thebandwidth offered to a facility should not be greater than the capacityof the facility for constant bit rate services such as voice. Each timea service request is made, the sequencer reads a virtual path cell entryfrom the list, loads the FSR with the header information that is storedin the list entry, and causes the CWB to load the 48 bytes into the FSR.

Active virtual paths are made inactive by removing entries from the listof active virtual paths associated with an FSR. The ATMU CC has a buffer921 it shares with the sequencer that the sequencer uses to add anddelete entries from active lists at idle times while cells are beingtransferred out of the FSRs.

The forgoing was described in a sequential manner. However, in reality,significant overlap of operations exist. For example, the checking ofFSR request bits overlaps with the accessing of previously checked FSRrequests, as well as with the sending of read orders to the CWB.

In the direction towards the SM (from the ATM network), cells areclocked into the FSRs from the LPUs, and request bits are set. The CLPservices these bits by first mapping the virtual path address in thecomposite cell header to a CWB location. This function is accomplishedby the ATM Address Mapper (AAM) 940 within the sequencer whichtranslates the ATM header address to a physical address in the CWBmemory devices. The AAM also uses the an index associated with the FSRso that the same ATM cell headers can be used on the differentfacilities, otherwise the headers would have to be different on allfacilities. Then the sequencer loads the cell in the FSR into the CWBlocation. All FSRs are on a parallel bus to the CWB, so that only onecan be loaded at a time into the CWB. In the preferred embodiment, theAAM is implemented using a Content Addressable Memory (CAM).

The design of the sequencer 920 is based on the use of high speed logic(programmable logic) that runs at over 100 MegaHertz (MHz) and scansshift registers request bits, operates the CAM, reads the linked list,gates data between the CWB and shift registers, and counts the cellarrivals. Examples of such logic are the PAL logic circuit familymanufactured by Advanced Memory Devices (AMD) that uses parts such asthe 22V10 device provided by many manufacturers. Another component isthe PLC14 by Signetics. Other gate array technologies exist from TexasInstruments that can also run 100 MHz plus.

ATM Congestion Test Function

One function of the ATMU is to be able to determine whether or not acomposite cell virtual path that is about to be placed into the activestate will experience congestion, and thus prevent such congestion fromoccurring. Congestion is defined as occurring if the utilization of thepath increases above some threshold along any segment.

Alternatively, when congestion occurs, cells are marked by one or moreATM-CMs along the path. The marking concept uses congestion control bitsin the ATM header to test the occupancy of links between the source anddestination. When the occupancy is above a threshold, the ATM-CM (andother intermediate ATM-CM) will mark the cell. The ATMU must note thisand report the event to the ATMU CC, which then reports the event to theSM. In order to provide a static indication, the cells should all arrivemarked over a period of a predetermined number of cell arrivals. Uponreception of a cell from an FSR that has request bits set, the CLP willtransfer the congestion control bits and the virtual path identifier toa memory that is accessible by the ATMU CC. The ATMU reads this memoryand reports the result to the SMP. If congestion occurs, new CBR PVCsare not activated if they use the congested link, and PVC consolidation(FIG. 22) is accelerated.

A diagnostic to test this mark detection mechanism is as follows: TheCLP is ordered by the ATMU CC to set the congestion bits on a link knownnot to be congested. Downstream ATM-CMs will not affect already setbits, so the far end ATMU should detect the set bits. The ATMU CC thenorders the bits to be cleared. The receiving ATMU should detect that thebits are cleared. Such a test can be used to verify the detectioncircuitry of the marked ATM cells, and the ability of the network totransport these bits after they are marked.

The exact use of the congestion bits has not been determined by theCCITT. In the arrangement described herein, these bits are used toverify bandwidth in a manner analogous to the frame marking algorithmused in Frame Relay. That algorithm drops frames that are marked ifcongestion occurs. However, ATM cells are not dropped since the markingthreshold is below the capacity of the channel.

The CLP is part of the same failure group as the space switch and CWB. Afault in the CLP causes a side switch of the ATMU, under control of theATMU CC.

Parity is used on all list and sequencer driven memories. Additionalhardware such as a timer that checks for sanity and dead clockconditions are also employed. The memory driven sequencer program isdownloaded by the ATMU CC as part of CLP initialization.

Control

The CLP is controlled by the ATMU CC. Registers exist on the CLP thatcause the CLP to modify information in the linked list table and/or theATM Address Mapper (AAM). In effect, the CLP simply writes ATMU CC datadirectly to those memories, but at a time that is synchronized withother CLP activities. The FSR request bits are inhibited by registersaccessible directly to the ATMU CC. Other communications such as markedcell information, or control to mark cells is handled through registers921 shared between the CLP and the ATMU CC. Any complex processing thatdoes not have to be done on a cell by cell basis is performed by theATMU CC.

In operation, after the SMP has connected a DS0 through a given SM MCTSIslice onto a NCT link, and has caused the ATMU central controller (ATMUCC) to connect the DS0 through the space switch into the correct virtualpath memory in the CWB, at the time a virtual path is activated, theATMU CC causes the virtual path address to be added to the list ofactive cells associated with the facility (FSR) the virtual path. Atthis point in time, the composite cells are transferred to the ATMnetwork, and DS0 continuity exists (at this end). The ATMU CC thenperforms various actions discussed above to test and monitor DS0continuity.

ATM Message Layer Device (FIG. 28)

The ATM Message Layer Device (MLD) (FIG. 28) provides variable lengthmessage to ATM conversion for the 5ESS switch for the following threecommunication systems:

SM to SM, and SM to AM communications

SS7 Network: SM to SM communications in the toll network

Inter-PSU packet communications

By providing these interworking services, the ATM network can be usedfor message transport, thereby simplifying inter-SM message exchangewithin in the 5ESS switch, and/or eliminating the use of STPs formessaging between switches in the toll network. STPs used for point ofpresence for local carders to the interexchange carrier are not affectedby this use of ATM. However, reduction of STPs brings about a savings inoperations cost and hardware costs.

Further, in accordance with the teachings presented herein, in thepreferred embodiment, the Message Switch within 5ESS switch (described,for example, in the AT&T Technical Journal reference on pages 1418-1421)is eliminated from the 5ESS switch, and the CM Processor Intervention(CPI) function is provided as discussed below in Section 4.6.

Contained within the MLD are the SS7 Inter-Working Unit (SS7 IWU) (1020)and the SM Inter-Working Unit (intra-switch IWU) (1022), and theinter-PSU Working Unit (1024) as shown in FIG. 17. A NCT bus from thespace switch of the ATMU transports one or more DS0 based channels thatcontain either SM or SS7 messages from the Message Handier and/or PSU tothe MLD. This NCT bus is duplex, having a source in each of theduplicated portions of the space switch. Within the MLD, the bus isfanned out (via backplane) to the SS7, SM, and inter-PSU IWUs.

The outputs of the MLD on the ATM side are serial shift registers thatare essentially in parallel with the shift registers of the CWB. Thiscan be seen in FIG. 18 which shows a block diagram for an IWU. The CLPservices requests from the MLD and interleaves the ATM based messagecells with composite voice cells. The algorithm for interleaving is thatcomposite cells are transmitted first (every 125 μs and then ATM basedmessage cells. The 125 μs interval should not be overlapped with thenext 125 μs interval for composite cells. The ATM based messages can beassumed to always have bandwidth on the facilities; facility usage isallocated in a way that guarantees that messaging bandwidth is alwaysavailable. Simulations and/or field experience are required to determinea minimum adequate margin for sending messages on facilities for whichcomposite cells are also transported.

In the incoming direction from the ATM network, the CLP translates ATMvirtual path headers, determines whether the cell is a SS7, a SM, or aninter-PSU message based virtual path, and directs the cells to thecorrect one of the IWUs within the MLD. The IWUs receive cells andcommence message re-assembly. Subsequently, the re-assembled messagesare transmitted in DS0s on the NCT bus to the space switch of the ATMU.The IWUs must be able to associate a cell from a given virtual path witha given DS0, or set of DS0s for N*DS0 pipes.

An alternative to the above would be to have the SM, SS7, and inter-PSUIWU connect to the ATM-CM directly via SDH/SONET facilities. However,this would use up limited facilities on the ATM-CM for traffic loadsthat are a tiny fraction of the facility's throughput. For this reasonthe slight overhead of having the CLP route the cells internally ispreferred. In addition, the DS0s must be multiplexed back into the 5ESSswitch SM, and the NCT buses and links are the most economicalarrangement for so doing.

An assumption is that at least one SM IWU is always equipped, since thatis the only mechanism an SMP has to communicate with the AM or otherSMPs. The SS7 IWUs are optionally equipped, so that they may or may notbe present, depending on the application. For example, in international5ESS switches, the SS7 links are often clustered in only one SM, andthis SM may direct all of its SS7 traffic to one ATMU. In addition, theMLD design allows for a variable number of SS7 Signaling Data Links(SDL). This may imply variable number of SS7 IWUs, depending on thenumber of SDLs that one SS7 IWU can handle. This flexibility helpspecialized applications such as the 5ESS switch's International'sGlobal SM (GSM), (the SM which has all the SS7 links). The GSM which hasa centralized SS7 PSU that can generate a large number of individual SS7SDLs, so that an ATMU that is able to terminate a significant number ofSDLs may minimize the software impact on existing 5ESS switches.

ATM and Protocols

The interaction of ATM and intra-switch and SS7 protocols is firstdiscussed and compared. A basic concept of ATM is that ATM providestransport level services only. To adhere to this concept, the SM IWUonly relays intra-switch frames. It does not perform protocol processingin the usual packet switching sense. For example, retransmissions arenot supported by the SM IWU. Protocol processors (i.e., PSU ProtocolHandlers and/or Message Handlers) process Link Access Procedures (type)D (LAPD) protocol end-end over the ATM network. In essence, the SM IWUidentifies the SM destination in an High-level Data Link Controller(HDLC) frame, assembles cells with the correct virtual path thatcorresponds to the destination SM, and then transmits the cells into theATM-CM.

The same arrangement is used for SS7. The ATM network relays SS7 MTPpackets between SS7 Signal Processors which are in the various switchesof the network. An SS7 implementation features a single point to pointlink (see CCITT standard Q.703) that usually connects to an STP, notused herein. However, the SS7 network is point to multi-point, at theMTP layer. To rationalize the point to point ATM layer with the point tomany point nature of MTP, the ATMU terminates the signaling link layer,and associates MTP packets with a virtual path to a destination thatcorresponds with the "MTP Point Code". The messages are assembled intocells, and then relayed over the ATM network. At the destination, theMTP packet is re-assembled, and a local signaling data link layer entityis used to send the SS7 packet back to the Message Handler or PSUProtocol Handler via DS0s on an NCT bus that connects to the spaceswitch (as was the case in the SM IWU). This approach helps minimize theimpact of the removal of STPs on a 5ESS switch or other switches sincethe ATMU essentially emulates the behavior of an STP (as viewed from theperspective of the PSU/PH) at the SDL level.

SM IWU

The function of the SM IWU is to associate LAPD frames with provisionedvirtual paths, and then convert the frames to cells. As stated above,the SM IWU does not execute procedural LAPD on the intra-switchmessages. However, the bit oriented LAPD protocol of the intra-switchmessages must be processed to verify message integrity via CyclicRedundancy Check (CRC), and more importantly, to remove bit stuffingfrom the message so as to be able to read the SM destination address.Only after the bit oriented protocol (BOP) is processed and the "bitstuffing" is removed can the SM destination address be read.

To accomplish the above, the SM IWU receives inter-SM LAPD messages on aprovisioned number of time slots (e.g., 12 DS0s or 768 kilobit/second)on the NCT bus from the ATMU space switch into an HDLC device 1120,(FIG. 18) on the SM IWU. The HDLC device stores the inter-SM frame in abuffer from which the address (and CRC check result) can be mad by theSM IWU internal controller. The controller associates the destination SMaddress with the correct virtual path. Then the frame with LAPD headeris assembled into ATM cells using the Connection Oriented NetworkServices (CONS) procedures in accordance with the ATM Adaptation Layerspecified in CCITT Standard I.363. As a brief summary, these proceduresinvolve the use of:

A bit field that indicates first segment (i.e., cell), middle segment,and last segment of the message

A CRC that also indicates the integrity of the overall message

A length field that indicates the number of bytes in the message

An index that binds all the cells of a given message together.

In the above list, the term "message" is used synonymously with LAPDframe. The assembly to cells is accomplished by a modified direct memoryaccess processor, the ATM Adaptation Layer Processor (AALP) thatgenerates the cells, bit fields etc. The output of the AALP is connectedto an LPU (FSR) that connects to the CLP. Currently only one FSR(connected to duplex LPUs) is supported by each IWU since the bandwidthfrom one facility is far beyond that which is necessary. When a cell isready for transfer, hardware in the CLP will transfer from the IWU FSRto the LPU. No cell wide buffer is needed in the outgoing direction, dueto the relatively low bandwidth requirements (as compared to thecomposite voice case).

In the reverse direction from the ATM network, the CLP reads the ATMheader in the FSR that connects to the LPU. That FSR receives cells forboth the composite memory (CWB) and MLD. At cell arrival, the CLP doesnot know if the cells are message based or composite. The CLP uses itsATM Address Mapper (AAM) (just as it would for composite voice) todetermine if the cell should be sent to an IWU, or to the CWB (compositevoice). For purposes of the immediately following discussion, the cellsare assumed to be message based, and destined for an IWU. Otherwise thecells would have been loaded into the (composite cell) CWB. The CLPmoves the cells to the SM IWU, or the SS7 IWU cell buffer 1170,(discussed below) or to the inter-PSU IWU. (This cell buffer 1170 is notthe CWB 620 used for the composite voice.)

The flow of signaling, control and other narrow band messages are anexample of data flowing from the ATM-CM to the SM (the opposite of thedirection illustrated in FIG. 29). The SM (intra-switch) IWU AALP copiesframes from the its cell buffer into queues that exist on a per messagebasis. Multiple message segments (cells) may be received concurrently,so that the identifiers in the cells are used to separate the cells intocomplete messages. After assembly, the messages are shipped back on DS0son the NCT bus through the space switch in the ATMU.

The SM IWU has a cell buffer because there must be sufficient bufferingto be able to receive a burst of cells if "cell bunching" occurs. Thismeans the "ATM front end" of the IWU must be able to receive cell burstsat facility rates for short periods of time. The cell buffer 1170 iscircularly organized (FIFO), and holds the entire ATM header plus 48bytes of data, unlike the CWB for composite cells which does not storethe ATM header. (In the composite cell case, the CLP processes anddiscards the ATM header for composite cells since the header serves nological function once the frame is in the CWB. In the message case, theATM header is further processed to assemble a complete message from theATM cells.)

Simulation studies and/or field experience can be used to determine theoptimum minimum depth of local cell wide buffering. Normally, thetraffic intensity is minimal compared to the throughput of the SM IWU,since the SM IWU only processes frames for one SM, and does not have toperform the processing to actually terminate the LAPD protocol as do itssources, the PSU Protocol Handlers or Message Handler.

The SM IWUs are spared on a simple duplication basis, since only one(plus its duplicate) are needed in an ATMU. This is because there areonly two inter-SM channels from the MH to the other SMs and AM.

Because the bandwidth required for inter-SM signaling channels is small,an NCT link carrying such signaling channels can be served by a singleIWU. The ATMU CC determines which SM IWU is active. The MLD NCT busfanout mechanism creates a cross coupling between the NCT buses from thespace switch, so that either SM IWU can be active regardless of whichspace switch side is active.

Faults are detected by parity checks on the NCT buses from the spaceswitch, as well as the internal error checking means normally used withprocessors and controllers, such as parity checks on memories and sanitytimers.

A separate type of fault tolerancy function regards message transportimpairment. Two types exist, ATM impairment, and Message Handler DS0channel impairment. Impairment means excessive cell or message errorrates. The SM IWU indicates message error conditions to the ATMU CC,such as corrupted frames from the MH/PSU, or corrupted messages from theATM network, if they exceed a certain threshold. This is detected ineither the ATM header, CRC-4, the CRC check in the LAPD frame from thePSU PH or MH, or the CRC within the message as it is transported withinATM Connection Oriented Network Service (CONS). In the event of high ATMerror rate, the ATMU CC can switch the SM IWU to the spare LPU (i.e., afacility protection switch) or take actions to diagnose the hardwarebetween the SM IWU and the LPU (including the SM IWU). In the lattercase, an ATMU side switch or SM IWU may be in order, depending on thecoupling between the MLD and the space switch.

SM IWU Control

The ATMU CC communicates with the SM IWU via a control output busconnected from the ATMU CC to many of the units of the ATMU. Inoperation, the ATMU CC provisions virtual path destination addresses viathis bus, and DS0s through the space switch into the IWU. The ATMU CCdoes this as directed by the SMP. Provisioning is also performed in theCLP to load the AAM with the virtual paths that carry the messages.After this is provisioned, inter-SM/AM intra-switch messages can be sentat will by the SMP/MH, without any ATMU CC intervention, to anyinter-SM/AM intra-switch destination.

SS7 IWU

The SS7 IWU is similar to the SM IWU, and occupies a position in the MLDthat is parallel to the SM IWU. Therefore this section will primarilyhighlight the differences between the SM IWU and the SS7 IWU. The majordifference between the SM IWU and the SS7 IWU are:

SS7 IWU terminates SS7 link protocol, but SM IWU only processes bitlevel LAPD to be able to read SM destination address

SS7 IWU is variably equipped, so that, as discussed above, it may or maynot be present. Furthermore, the SS7 IWU allows a variable number ofSignaling Data Links to be supported, in order to allow a 5ESS switchGlobal SM to be supported on one ATMU.

As stated in the section on ATM and Protocols, the SS7 IWU terminatesthe SS7 link (SDL, level 2) protocol as received from the PSU ProtocolHandlers or Message Handlers on the NCT bus between the space switch andthe MLD. The SS7 IWU then associates a virtual path with the MTP pointcode, assembles the packets into cells, and transfers them out to theATM network. At the destination switch, the CLP routes the cells to theSS7 IWU which then assembles them back to packets, inserts them into aSS7 signaling data link frame, and sends them back on the NCT bus to thespace switch towards the PSU Protocol Handlers or Message Handler. TheATM network is used only as a transport mechanism for MTP packets as wasthe case for the intra-switch messages processed by the SM IWU.

The design of a SS7 IWU and a SM IWU is very nearly identical. The NCTbus side of the IWUs are both HDLC based, and controllers that processBit Oriented Protocol (BOP) can also process the SS7 SDL protocol. AT&Tmanufactures a controller set (2 devices, the ATT 7115 and ATT 7130)that perform this function for 32 channels with all channels at fullutilization for SS7 SDL. The ATM front end and interaction with the CLPis likewise identical, and will not be repeated here. Primarily, the MTPPoint Code is used in place of the SM destination for purposes ofvirtual address association.

In one alternative architecture, the SS7 and SM IWU are located on thesame IWU. Throughput is not an issue, only the number of DS0s to besupported on one hardware card. Given the NCT bus size of 512 timeslots, and the use of double size cards, more than 32 time slots may besupportable in one card, making the use of a single IWU attractive incost.

The SS7 IWU has a fault tolerancy design different from the SM IWU.Because a variable number of SS7 IWUs are used in one ATMU, the SS7 IWUredundancy in the MLD is N+1, versus 1+1as in the SM IWU. In analternative arrangement, the redundancy is N+2.

Control

The SS7 IWU has similar control issues as does the SM IWU. The SMPprovisions:

Time slots from the Message Handler or PSU Protocol Handlers through thespace switch to the SS7 IWU

Maps in the SS7 IWU that associate Point Codes with virtual paths

Virtual paths in the CLP.

Inter-PSU IWU

The Inter-PSU IWU works in essentially the same way as the inter-SM IWU,except that it switches packets supplied by PSU 519, packets which, ingeneral, originate from and terminate to users. The IWU is transparentto the user level protocol being used.

ATMU Control

The NCT links connect the ATMU to the SM. Therefore, the same type ofcontroller that is used in a digital trunk unit (DTU) or SONET InterfaceUnit (SIU) is used in the ATMU.

The ATMU CC operates and maintains the ATMU hardware as discussed in allthe above sections, and therefore will not be repeated here.

The ATMU CC is 1+1 spared., and is in a separate failure group from therest of the ATMU. Since the ATMU CC is a modified version of the SIU/DTUcontroller, no further discussion of the fault tolerancy aspects of thecontroller is required.

In the 5ESS switch, a protocol handler (message handler (MH)) is used tointerface between the SMP and the TSI; control and signaling messagescan then be transmitted between the TSI and the ATMU's MLD. This MessageHandler in the 5ESS switch is used to communicate with the ATMU CC. Thiscommunication is via time slots that arrive on the links, and that carryLAPD based messages which ultimately originate from the SMP. The ATMU CCprocesses the LAPD, and executes the commands. Certain special functionssuch as processor reset are put into the special NCT link time slot usedto transmit control messages to the ATMU CC if ATMU CC sanity is lostaltogether for either side. Special bits in a predesignated controlchannel are reserved on the the NCT link to implement these functions.

As previously discussed, the ATMU can be located within the ATM-CM, inwhich case the ATMU CC is controlled by a controller of the ATM-CM.

CBR Processor Intervention (CPI)

The SMP Intervention capability that the AM uses to reset an insane SMPwill now be discussed. In the current 5ESS switch, the AM controlsmessage switch hardware to source bits in the Control Time Slot (CTS) ofthe NCT link. Since a goal of this design is to eliminate the MessageSwitch, the capability must be emulated by some combination of theATM-CM and the ATMU. This capability is implemented as follows:

Special virtual paths from the AM to the ATMU carry cells withmulti-byte patterned data. These are repeatedly sent to the ATMU by theAM via the APH 1240 to perform CPI. ATMU hardware (the Facility ShiftRegisters) is able to detect the condition, and is hard wired to thespace switch, which then generates the CTS CPI bit pattern to reset theSM. This multi-byte pattern data can be a long pseudo random sequencethat has essentially zero probability of ever being generated, withincells that should never be sent except to reset a specific SM. TheATM-CM is provisioned with these virtual paths between the AM andindividual SMs.

AM and ATM-CM Operations, Administration, Maintenance, and Provisioning(OAMP) Platform

The AM serves to support the entire 5ESS switch and ATM-CM (includingATMUs) OAMP needs. These include download and control of the ATM-CM,craft graphical display, and communication via ATM with SMs. FIG. 19shows the AM/ATM-CM system architecture as comprising the followingcomponents:

ATM Management Module (AMM) including directly connected terminal. Thisis an adjunct fault tolerant processor that is an extension of theexisting 5ESS switch AM, and serves to provided added processingthroughput for new ATM-CM and ATMU capabilities.

Ethernet Bus to interconnect AM/AMM with Graphic Unit Interface (GUI),ATM Packet Handler (APH), and ATM-CM.

Small Computer System Interface (SCSI, an industry standard) peripheralsfor disk, tape, and CD ROM on-line documentation: These augment theexisting AM non-volatile peripherals.

GUI workstation terminals that supports existing 5ESS switch equipment,ATM-CM, and ATMUs.

ATM Packet Handler provides the AM/AMM with the ability to communicatevia ATM over SONET to the SMs. The SMs terminate the APH's ATM in theirATMU MLD's. To communicate with SMs, the AM/AMM sends messages viaEthernet to the APH which performs the message to cell conversion andtransmittal to ATM-CM over SONET.

The GUI and non-volatile memory are commercial components whose controlresides in AMM software. The design of the AMM and APH components areexpanded in the following sections.

ATM Management Module

The AMM is a high capacity processing element of the AM:

Processors: AMM The processors are N+K redundant connected by a FutureBus(+) technology, IEEE 896 Standard. Automatic hardware and softwarefault detection, and restart capabilities are built into the processormodules. The AMM itself is supported by the Administrative Module (AM)which is used to create a high reliability environment for the AMMprocessors.

Memory Modules: The N+K processors share common memory modules that areused to store static and check-pointed data. Automatic hardware errordetection capabilities are built into the memory modules. The memorymodules are redundant, with data being stored in two memory modules;only the active module responds to read accesses. The memory modulesconnect to the processors via Future Bus(+).

SCSI Peripheral Interface: There are two SCSI controllers in theAM/ATM-CM Platform. In addition to mirrored disks, there is a cartridgetape drive for loading the AMM and a CD ROM is optionally equipped tostore on-line documentation.

Ethernet Transceivers: The ATM-CM and GUI workstations are connectedthrough duplicated Ethernet interfaces to the AMM.

Dedicated AMM Terminal: A terminal directly to the processor complex, inorder to access the core in the event both terminal controllers orEthernet transceivers fail. This terminal does not have a GUI, and isintended for Man Machine Language (MML) commands only.

APH

The APH is a modified IWU from the ATMU MLD. It is shown in FIG. 30. TheRate Adapt and BOP controller are removed and are replaced by anEthernet Controller and Transceiver. The Ethernet controller placesmessage into the buffer. The processor determines the proper ATM header(Virtual Circuit). The AALP performs the functions of convening themessage to cells. Unlike the MLD IWU, the Cell Buffer connects directlyto an LPU that transforms bits to SONET media as soon as the segmentsare available. There is no CLP to gate the transmission of the cell tothe LPU.

CCR

FIGS. 42-45 are four configurations of the CCR. While the preferredembodiment relates to ATM signals, a more general version of a CCR is acomposite packet remap (CPR) unit, which remaps contents of compositepackets in any packet system.

FIG. 42 is for a CCR handling cells with 48 single byte DS0s per cell,both in and out. E-bits are simply not involved here. An example of the48-byte to 48-byte mode is in the use of the CCR as shown on FIG. 11,where the CCR connects two transit ATM crossconnects, and in the centralCCR of the example of FIG. 4. The input as previously noted, comprises53 byte cells, each cell containing 48 single byte segments and a 5-byteheader. A demultiplexor (demux 4002) receives the input and transmitsthe 48 bytes to the data rearrangement block 4010 and the 5-byte headerto the ATM header processor 4006. The ATM header processor 4006 deliversthe virtual circuit identifier (VCI) and virtual path identifier (VPI)to address generator 4020, shown in detail on FIG. 46. The addressgenerator 4020 generates addresses for the RAM 4014 which receives the8-bit bytes from the input stream and delivers 8-bit bytes at itsoutput. Between the demux 4002 and the RAM 4014 is a serial-to-parallelconverter 4012 for generating sequentially the 48 byte wide inputs toRAM 4014. Between the RAM 4014 and the multiplexor (mux) 4004 is aparallel-to-serial converter 4016 to take the single byte output of theRAM and generate a serial bit stream. Mux 4004 also receives input fromthe ATM header assembler 4008 which represents the 5-byte header of theoutgoing cell.

FIG. 43 represents a CCR for receiving 42 segments of data in each cell(the 42 segments representing each one PCM sample byte and one E-bit)and delivering 48 bytes of data per cell. An example of the use of theconfiguration of FIG. 43 is for a CCR which is receiving inputs from anATMU (switched through an ATM-CM without affecting the contents of anyof the payload of the cell) and which is transmitting cells to an ATMcrossconnect. The left CCR of FIG. 3 performs this function. Thedifferences between FIGS. 42 and 43 are that the serial-to-parallelconverter 4012 delivers every ninth bit of the input stream to E-bitprocessor 4022. The E-bit processor records the E-bit associated witheach write address in its own internal RAM (not shown) and is able todetect a change in the received E-bit for that address. When the changeis detected, the external control is notified of a change insupervision.

FIG. 44 is for a CCR whose input comprises cells having 48 bytes of dataper cell, each byte representing one PCM sample and whose outputcomprises cells having 42 segments of data (each segment including aone-byte PCM sample and an E-bit). The right CCR of FIG. 3 performs thisfunction. The E-bit is inserted in every ninth position of the payloadthat is generated for the output by parallel-to-serial converter 4016.The configuration of FIG. 44 includes an E-bit generator 4024 whichcontains a RAM (not shown) with E-bit values to be applied for each readaddress. The contents of the E-bit RAM are updated from the externalcontrol.

Finally, FIG. 45 shows a CCR for accepting a signal stream comprisingcells carrying 42 segments of input data per cell and delivering 42segments of output data per cell, each segment including an 8-bit PCMsample and a single E-bit. The CCR of FIG. 2 performs this function. Theonly difference between the configuration of FIG. 45 and that of FIG. 42is that the RAM is one segment wide (9 bits) instead of being one bytewide (8 bits). Alternatively, it is possible to provide a 42-bit to42-bit CCR which terminates the input E-bit on an E-bit processor andgenerates a new E-bit using an E-bit generator. This has the advantageof terminating the section of a call at the CCR. The configuration ofFIG. 45 would be used for any cases in which it was found desirable toswitch ATMU to ATMU connections within a common access switch through aCCR; the E-bit (and probably other comparable bits) are not normallytransmitted out of an access switch.

FIG. 46 shows details of the address generator 4020. Input configurationRAM 4030 is used to address RAM 4014 for writing new inputs into thatRAM the new inputs being the data from 8-bit bytes or 9-bit segmentsfrom the ATM input cell stream. In this preferred configuration, theinput is stored in RAM locations arranged arbitrarily, and the output isread sequentially one byte or segment (FIG. 45) at a time. The inputVPI/VCI (from the header) is examined by the cell address translator4028 to determine an internal cell address in the range of 1-180 for thecell. The VPI and/or VCI can be directly coded to indicate this celladdress or an arbitrary VPI/VCI value can be used if a translationcontent addressable memory (CAM) is incorporated into the cell addresstranslator. (Note that the VPI and VCI together are 28 bits long so thatthe use of an indexed memory is out of the question.) The inputconfiguration RAM 4030 is addressed by the cell ID and by a segmentcounter 4032 which counts from 1 to 48 and is reset at the beginning ofeach cell after the cell header has been received.

The output configuration RAM 4040 is driven by the cell counter 4042 andbyte counter 4044. In addition, the cell counter drives the outputheader store 4046 which contains the output virtual path identifier andvirtual circuit identifier and which serves as one of the inputs of Mux4004 (FIG. 45). The output configuration RAM 4040 then drives RAM 4014whose output is delivered to a parallel-to-serial converter 4016 whichdelivers 8 byte data into Mux 4004. The cell and byte counter are resetat an 8 KHz rate.

Alternatively, a 48 byte wide output could be delivered from the RAM4014, driven only by the cell counter 4042; this would halve theoperating speed of the RAM, but would require 48×8 output circuits inthe RAM 4014 (instead of 8). While the preferred embodiment shows twoaddressing memories, alternatively, either one could be replaced by acounter so that the CCR could work by having sequential load, random(i.e., control memory directed) unload, or random load sequentialunload.

For the case of FIG. 43, the CCR receives from the ATM-CM composite CBRcells containing 42 9-bit segments each comprising an 8-bit PCM sampleand a single E bit, and transmits composite CBR cell outputs and whicheither goes directly to another CCR or to a transit ATM crossconnectcontain 48 8-bit PCM samples in each cell. At first glance this mayappear to be wasteful. However, it is important to remember that theinput cells and the output cells are frequently not fully packed. Sincethe CCR performs rearrangement of data within the cells, it willfrequently happen that there is an imperfect mapping between the numberof cells required at the output and the number of input cells. Thissituation is greatly alleviated if the output cells can transmit moresamples than are received in each input cell. If the maximum number ofsamples in each input cell were to equal the maximum number of samplesin each output cell, blockage would be likely to occur during periods ofpeak traffic. With this arrangement, blockage is sharply reduced.

As mentioned earlier, the cell layout of FIG. 17 is the present standardfor communicating voice signals using ATM. While this arrangement doeshave the disadvantage of introducing a 6-millisecond delay required toaccumulate the samples for each packet, it may be necessary at leastinitially to communicate with units which observe this standard. When itis necessary to create such cells, the CCR is ideally equipped to do so.All that is required is to expand the size of the counter by a factor of48:1 and to expand the size of the memory by the same factor, so that 48times as many different cell addresses may be accommodated (for the casein which all of the output traffic is of the FIG. 17 format). The cellID translator 4028 similarly must be expanded from one to 180, to one to48 times 180 and the CAM of that identifier must be similarly increasedin size. For generating the standard format composite cell, 48 frames ofinputs must be stored, requiring 48*180 cells of storage whereas for thecomposite cell described elsewhere herein, only one, two or three (inorder to handle jitter) frames of data need to be stored. Once thecounter, memory and cell ID translator have been expanded, the CCRperforms its remapping function in the same way as previously described.

The CCR can also be used to transmit wideband data transparently since,while it is capable of performing the remap function, it is also capableof executing the remap function by simply taking the contents ofincoming cells and generating them as outgoing cells without changingthe payload contents. Thus, a CCR can be used in conjunction withperforming a transit switching function on the standard format compositecells and importantly, can handle a mix of standard and proposed formatcells.

Call Scenarios

This section describes the use of broadband switching of composite cellsbetween and originating and terminating ATM-CM offices. To facilitatethe examples, it is assumed the two 5ESS offices provide a tandem callconnection between two LEC offices whose access is T1 and N-ISUP, asshown in FIG. 21. Also, for the sake of simplicity, the scenarios inthis section assume that the ATMU is part of the ATM-CM.

Below is a high-level scenario for an incoming narrowband ISUP tooutgoing broadband ISUP call in an originating switch. It is followed bythe corresponding incoming broadband ISUP to outgoing narrowband ISUPscenario in the terminating switch. Both a message diagram and adescription of the tasks for each message is included. Other scenarioswith various combinations of broadband and narrowband accesses and trunkhave been derived, but are not included here.

The call flows are based on the current 5ESS International trunk-trunkcall model. A similar model can be derived for US 5ESS.

The following conventions are use in the line diagrams:

solid lines with upper case font represent external messages,

solid lines with lower case font represent internal messages,

solid lines with italic font represent hardware register read/writes,

dashed lines represent composite cells,

dotted lines represent NCT information (i.e., E-bits).

NISUP-BISUP Call Scenario - Originating ATM-CM Switch (FIGS. 34-37) ofan Originating Call

This section describes the implementation for one preferred embodiment.

1. The originating narrowband call control processing in the SM receivesthe incoming Initial Address Message (IAM) and performs digit analysis.The IAM is sent to the terminating switch. If there are intermediatecrossconnects, the IAM is passed through without processing. If thereare intermediate tandem switches, the tandem switch processes the IAMand forwards an IAM toward the terminating switch. The originating SM isone such intermediate tandem switch for a call originating in a localswitch 2 (FIG. 6). The IAM arrives at the originating SM via the normalnarrowband signaling processor, a packet switch unit, or, for the AT&Tnetwork, a CNI ring. Digit analysis results indicate that the call doesnot terminate on another narrowband trunk served by this switch, butinstead, must be routed to another node in the toll network. The digitanalysis results would identify the node in the network the call shouldbe routed to.

2. N-ISUP call control in the SM sends a routing request to the AM overthe NCT link as normal.

3. Based on the results of initial routing in the SM, the AM recognizesthe call as an inter-switch call to be transported via ATM. The AMselects a timeslot on the originating NCT link, and requests pathhunting and setup from Connection Control in the ATM-CM.

Connection Control in the ATM-CM maintains all the status informationregarding VPs/VCs between local SMs, CCRs, and other nodes in thenetwork. Connection Control is also responsible for maintaining thecomposite cell mapping information for all ATMUs and CCRs in the office.Data is kept which associates SDCC/MDCC bytes with DS0 timeslots (a copyof which is kept in the ATMU), and of MDCC-to-TDCC (Tandem DestinationComposite Cell) remapping (copied into the CCR). In addition, ConnectionControl maintains data which associates destinations in the network withactive and inactive VPs to those nodes, and also the CCRs associatedwith those VPs.

Connection Control determines an outgoing VP based on the destinationnode. Connection Control may have several VPs to chose from to reach thedestination node, and would select an active VP with a TDCC withavailable slots. If an active composite cell VP is available, ConnectionControl reserves a slot in the composite cell. If no active compositecell VP exists, Connection Control activates a new VP if bandwidth(i.e., space for an additional CBR cell per frame) is available. If nobandwidth is available, Connection Control determines an alternate VP.The alternate VP may not be a direct VP to the destination node, but maybe a VP which is routed via a switch at another node. Connection Controlalso determines the signaling VP associated with the composite cell VPto be used for the call.

Connection Control then allocates a slot in the MDCC between theoriginating ATMU and the CCR. If no active MDCC exists, a new one isactivated.

4. Connection Control updates the timeslot-to-MDCC mapping informationin the ATMU.

5. Connection Control updates the MDCC-to-TDCC remapping information inthe CCR. The CCR takes MDCCs from many ATMUs with calls destined for thesame destination to be remapped (multiplexed) into one (or as many asneeded) TDCCs to the destination node.

6. Connection Control sends a termination request message to B-ISUP callcontrol in the BB-CP. Broadband call control is involved because thecall is going out of the office on a broadband facility (broadband ISUP)to another node in the network. Broadband call control acts as theoutgoing half call. The message contains the VPI/VCI (Virtual PathIdentifier/Virtual Circuit Identifier) and NCT timeslot to be used forthe call. It also identifies the outgoing signaling VP/VC and DS0 byteto be used.

7. The BB-CP performs Broadband ISUP (B-ISUP) call control. B-ISUP callcontrol formats and sends the IAM to the terminating office via BB-SP.BB-CP performs all ATM Adaptation Layer (AAL), MTP layer 3 signaling,and Global Title Translation (GTT) for broadband signaling. All SS7signaling VCs to other switches terminate on the BB-SP.

The IAM contains the VPI/VCI and DS0 cell byte position. The BB-CPserves as the "terminating half-call" for the call.

8. B-ISUP in the BB-CP sends a "path₋₋ close" message to the ATM-CM.

9. For a 5ESS switch, the ATM-CM updates the CCR in order to turn on theE-bit for the call in the MDCC between the CCR and the ATMU.

10. At the ATMU, the E-bit in the MDCC is mapped to the appropriatetimeslot.

11. B-ISUP in the BB-CP sends a "setup complete" message to theoriginating SM. The message identifies the timeslot selected byConnection Control in the ATM-CM to be used for the call.

12. In a 5ESS switch, when N-ISUP call control in the SM detects E-bit,and receives the "setup complete" message, it sends E-bit on theselected timeslot.

13. In a 5ESS switch, the ATMU takes the received E-bit and maps it intothe MDCC to the CCR.

14. In a 5ESS switch, the ATM-CM detects a change in the E-bit for thecall from the CCR, and sends a "path₋₋ set" message to the BB-CP. B-ISUPcall control in the BB-CP completes setup of the call.

15. The ACM is received from the far end switch.

16. B-ISUP call control in the BB-CP sends a "buildup complete" messageto N-ISUP call in the originating SM.

17. N-ISUP call control in the SM sends an ACM message to the narrowbandLEC switch.

18. The Answer Message (ANM) is received from the far end switch.

19. B-ISUP call control in the BB-CP sends a "answer charge" message toN-ISUP call control in the SM.

20. N-ISUP call control in the SM sends an ANM message to the narrowbandLEC switch. The call is now in the talking state.

Assuming originating party disconnect, the call clearing scenario is asfollows:

1. A narrowband ISUP RELEASE message is received by N-ISUP call controlin the SM. N-ISUP call control begins clearing the call by sending a"release" message to the far half-call (i.e., the BB-CP), in a 5ESSswitch, turning off E-bit on the timeslot, and sends a RLC message tothe narrowband switch to complete the ISUP disconnect sequence.

2. In a 5ESS switch, the ATMU maps the timeslot E-bit into theappropriate MDCC to the CCR.

3. In a 5ESS switch, the ATM-CM detects E-bit discontinuity in the CCR,and sends a "path release" message to the BB-CP.

4. B-ISUP call control in the BB-CP clears its end of the call bysending a clearing message to the ATM-CM in order to release the SMtimeslot and the virtual resources (VCs and/or composite cells) in theATMU and CCR, and also begins the REL/RLC (Release/Release Complete (SS7signals)) sequence with the far end.

5. The ATM-CM updates the composite cell mapping tables in both the ATMUand CCR.

BISUP-NISUP Call Scenario-Terminating ATM-CM Switch (FIGS. 38-41)

1. The incoming broadband IAM is routed via the BB-SP to broadband callcontrol in the BB-CP. All SS7 signaling VCs to other switches terminateon the BB-SP.

The incoming BISUP call control processing in the BB-CP performs digitanalysis. Digit analysis results indicate that the terminating trunkgroup is served by this switch, and that it is a narrowband trunk group.Contained in the IAM is the identity of the virtual path used for thecomposite cell, as well as the identity of the byte used in thecomposite cell. This identifies the CCR which will be used for the call.

2. Based on the results of initial routing in the BB-CP, BISUP callcontrol sends a routing request to the AM. Included in the routingmessage is the composite cell information received in the IAM.

3. The AM performs a trunk hunt, determines the destination SM, andselects a timeslot. The AM requests network path setup from the ATM-CM.Included in the request is the composite cell information received inthe IAM.

4. As described in the previous scenario, Connection Control in theATM-CM maintains all the status information regarding VPs between localSMs, CCRs, and other nodes in the network.

Based on the terminating SM and the composite cell information,Connection Control determines if a MDCC virtual path is currently activebetween the ATMU serving the terminating SM and the incoming CCR. If noactive virtual path exists (or if all cells on existing paths are full)Connection Control activates a new MDCC virtual path. Connection Controlallocates a byte in the MDCC to be used for the incoming call.

5. Connection Control updates the timeslot-to-MDCC mapping informationin the ATMU.

6. Connection Control then updates the TDCC-to-MDCC remappinginformation in the CCR.

7. Connection Control sends a path acknowledgement message back to theAM.

8. The AM sends a termination request to the destination SM. The messagecontains the identity of the selected trunk, and the timeslot selectedby the IAM.

9. N-ISUP call control in the SM formats and sends the IAM to theterminating LEC switch and sends E-bit on the NCT.

10. In a 5ESS switch, the ATMU in the ATM-CM takes the received E-bitand maps it into the composite cell to the CCR.

11. In a 5ESS switch, the CCR in the ATM-CM detects a change in theE-bit for the call, and sends a "path₋₋ set" message to the BB-CP.

12. N-ISUP call control in the SM sends a "setup complete" message tothe BB-CP. The message identifies the timeslot selected by ConnectionControl in the ASU-CP to be used for the call.

13. When B-ISUP call control in the BB-CP receives the "setup complete"and "path₋₋ set" messages in a 5ESS switch, it requests the ATM-CM toset the E-bit in the composite cell to the ATMU.

14. In a 5ESS switch, the ATM-CM updates the CCR in order to turn on theE-bit for the call in the MDCC between the CCR and the ATMU.

15. In a 5ESS switch, at the ATMU, the E-bit in the MDCC is mapped tothe appropriate NCT timeslot.

16. In a 5ESS switch, N-ISUP call control in the SM detects the E-bitand completes the path setup.

17. The ACM is received from the LEC switch.

18. N-ISUP call control in the SM sends a "buildup complete" message toB-ISUP call control in the BB-CP.

19. B-ISUP call control in the BB-CP sends an ACM message to the far endATM-CM switch.

20. The ANM is received from the LEC switch.

21. N-ISUP call control in the SM sends a "answer charge" message toB-ISUP call control in the BB-CP.

22. B-ISUP call control in the BB-CP sends an ANM message to the far endATM-CM switch. The call is now in the talking state.

Assuming A-part disconnect, the call clearing scenario is as follows:

1. A broadband ISUP RELEASE message is received by B-ISUP call controlin the BB-CP. B-ISUP call control begins clearing the call by sending aRLC message to the far end ATM-CM switch to complete the broadband ISUPdisconnect sequence, and by sending a "release" message to the SM.

2. B-ISUP call control in the BB-CP sends a reset E-bit message to theATM-CM in order to turn off the E-bit in the composite cell to the ATMU.

3. The terminating ATMU maps the E-bit change in the MDCC to theterminating NCT link.

4. N-ISUP call control in the SM detects E-bit discontinuity, and beginsthe N-ISUP REL/RLC sequence with the terminating LEC switch.

5. N-ISUP call control in the SM also sends a timeslot release messageto the ATM-CM in order to release all network path resources.

6. The ATM-CM updates the mapping table in the ATMU and CCR involved inthe call, and also releases the timeslot.

As previously stated, where two units communicating using ATM cells arein close physical proximity, it should not be necessary to use a SONETor SDH facility to communicate these signals.

While this description has used voice as the primary narrowband signalexample, narrowband data (64 KB or less), facsimile and other narrowbandsignals can be switched in the same way.

It is to be understood that the above description is only of onepreferred embodiment of the invention. Numerous other arrangements maybe devised by one skilled in the art without departing from the scope ofthe invention. The invention is thus limited only as defined in theaccompanying claims.

    ______________________________________                                        APPENDIX A                                                                    ACRONYMS AND ABBREVIATIONS                                                    ______________________________________                                        AAL     ATM Adaptation Layer                                                  AALP    ATM Adaptation Layer Processor                                        AAM     ATM Address Mapper                                                    AM      Administrative Module                                                 AMD     Advanced Memory Devices                                               AMM     ATM Management Module                                                 ANM     Answer Message                                                        AP      Administrative Processor                                              APH     ATM Packet Handler                                                    ASU     ATM Switching Unit                                                    ATM     Asynchronous Transfer Mode                                            ATMU    ATM Interface Unit                                                    ATMU CC ATMU Central Controller                                               BB      Broad Band                                                            B-ISUP  Broadband ISUP                                                        BOP     Bit Oriented Protocol                                                 CAM     Content Addressable Memory                                            CBP     Common Broadband Platform                                             CBR     Constant Bit Rate (traffic)                                           CCB     Composite Cell Byte                                                   CCITT   Consultative Committee on International Telephone                             and Telegraph Standards                                               CCR     Composite Cell Remap                                                  CD ROM  Compact Disk Read Only Memory                                         CLP     Cell List Processor                                                   CM      Communication Module                                                  CNI     Common Network Interface                                              CONS    Connection Oriented Network Services                                  CPI     CBP Processor Intervention                                            CPR     Composite Packet Remap                                                CRC     Cyclic Redundancy Check                                               CTS     Control Time Slot                                                     CWB     Cell Wide Buffer                                                      DACS    Digital Access Crossconnect System                                    DCC     Digital Communications Channel                                        DS0     A 64 kilobit/second PCM single channel signal                         DS1     A signal composed of 24 DS0 signals                                   DTU     Digital Trunk Unit                                                    EOC     Embedded Operations Channel                                           FAX     Facsimile                                                             FSR     Facility Shift Register                                               GSM     Global SM                                                             GTT     Global Title Translation                                              GUI     Graphical User Interface                                              HDLC    High-level Data Link Controller                                       HDTV    High Definition Television                                            IAM     Initial Address Message                                               ISDN    Integrated Services Digital Network                                   ISUP    ISDN User Port                                                        IWU     Inter-Working Unit                                                    LAPB    Link Access Procedures (type) B                                       LAPD    Link Access Procedures (type) D                                       LEC     Local Exchange Carrier                                                LPU     Line Processing Unit                                                  MCTSI   Module Controller Time Slot Interchange                               MDCC    Multiple Destination Composite Cell                                   MHZ     Megahertz                                                             MLD     Message Layer Device                                                  MML     Man Machine Language                                                  MTP     Message Transfer Part                                                 N-ISUP  Narrow Band ISUP                                                      NCT     Network Control and Timing (link)                                     NLI     NCT Link Interface                                                    NNI     Network Node Interface                                                OAMP    Operations Administration Maintenance and                                     Provisioning                                                          OSS     Operation Support System                                              PCT     Peripheral Control and Timing (link)                                  PH      Protocol Handler                                                      PSU     Packet Switching Unit                                                 PVC     Permanent Virtual Circuit                                             RASU    Remote ATM Switching Unit                                             REL/RLC Release/Release Complete (SS7 signals)                                SCCP    Signaling Control and Connection Part                                 SCSI    Small Computer System Interface (an Industry                                  standard)                                                             SDCC    Single Destination Composite Cells                                    SDH     Synchronous Digital Hierarchy                                         SDL     Signaling Data Link (SS7)                                             SIU     SONET Interface Unit or SM Interface Unit                             SM      Switch Module                                                         SMDS    Switched Megabit Data Switch                                          SMP     Switch Module Processor                                               SONET   Synchronous Optical NETwork                                           SP      Signaling Processor                                                   SR      Shift Register                                                        SS7     Signaling System (Number) 7                                           STM     Synchronous Time Multiplex                                            STP     Signal Transfer Point                                                 STS     Synchronous Time Signal                                               TDC     Tandem Destination Cell                                               TDCC    Tandem Destination Composite Cell                                     TSA     Time Slot Assign                                                      TSI     Time Slot Interchange                                                 TSIU    Time Slot Interchange Unit                                            UNI     User Network Interface                                                VBR     Variable Bit Rate (traffic)                                           VC      Virtual Circuit or Virtual Channel                                    VCI     Virtual Circuit Identifier or                                                 Virtual Channel Identifier                                            VP      Virtual Path                                                          VPI     Virtual Path Identifier                                               ______________________________________                                    

We claim:
 1. An access switch for generating packetized signals fordistribution by a packet signal transmission/crossconnect network, saidnetwork comprising means for interconnecting a plurality of saidpacketized signal streams without disturbing the payload contents ofcells of said packetized signal streams, the access switch comprising:aplurality of CPR (composite packet remap) units; means for convertingmultiplexed periodic communication input signals, comprising periodiccommunication input channels, different ones of which are fortransmission to different ones of a plurality of switches accessed viasaid plurality of CPR (composite packet remap) units, into packetizedsignals, said packetized signals each comprising first packets each fortransmitting data for a plurality of separate communications, each firstpacket for transmission to a single CPR unit of said plurality of CPRunits; packet switching means for switching each of said first packetsto one of said plurality of CPR units without changing the payload ofthe switched first packets; and said plurality of CPR units each forreceiving said first packets and rearranging payload data of said firstpackets to generate an output stream comprising second composite packetseach second packet containing payload data destined for a single CPRunit of one other access switch for switching in said one other accessswitch to one of a plurality of multiplexed periodic communicationoutput signals comprising periodic communication output channels.
 2. Theaccess switch of claim 1 wherein said multiplexed periodic communicationsignals are PCM (pulse code modulation) signals.
 3. The access switch ofclaim 1 wherein said packetized signals are ATM (asynchronous transfermode) signals, and said composite packets are composite cells, and saidplurality of CPR units are a plurality of CCR (composite cell remap)units.
 4. The access switch of claim 3 wherein said packet switchingmeans comprise an ATM packet switch.
 5. The access switch of claim 4wherein said first packets are transmitted through said ATM packetswitch through a virtual path through said ATM packet switch.
 6. Theaccess switch of claim 4 wherein said multiplexed periodic communicationsignals are PCM (pulse code modulation) signals.
 7. The access switch ofclaim 6 wherein said means for converting comprises a plurality of ATMU(asynchronous transfer mode unit) units each for receiving at least onesignal stream comprising PCM samples and for generating at least one ATMsignal, said at least one ATM signal comprising said PCM samples of saidat least one PCM input stream, said at least one ATM signal comprisingcells, each cell of said at least one ATM signal for transmission to oneof the plurality of CCR units of said access switch.
 8. The accessswitch of claim 7 wherein said each access switch comprises an ATM-CM(ATM communications module) for distributing cells from said at leastone ATM signal of said at least one ATMU unit to ATM input streams ofsaid at least one CCR unit.
 9. The access switch of claim 8 wherein saidat least one ATM signal generated by an ATMU unit further comprisesadditional cells, said additional cells for transmission to one of theATMU units of said access switch.
 10. The access switch of claim 9wherein said ATM-CM further distributes said additional cells to ATMinput streams of one of the ATMU units of said access switch.